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14 of the Most Baffling Mysteries About the Universe

Updated: Aug. 22, 2023

We've learned so much over the centuries, but science still can't definitively answer these surprisingly fundamental questions.

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Observations by the NASA/ESA Hubble Space Telescope have taken advantage of gravitational lensing to reveal the largest sample of the faintest and earliest known galaxies in the universe. Some of these galaxies formed just 600 million years after the big bang and are fainter than any other galaxy yet uncovered by Hubble. The team has determined for the first time with some confidence that these small galaxies were vital to creating the universe that we see today. An international team of astronomers, led by Hakim Atek of the Ecole Polytechnique Fédérale de Lausanne, Switzerland, has discovered over 250 tiny galaxies that existed only 600-900 million years after the big bang— one of the largest samples of dwarf galaxies yet to be discovered at these epochs. The light from these galaxies took over 12 billion years to reach the telescope, allowing the astronomers to look back in time when the universe was still very young.
Courtesy NASA/ESA

What came before the Big Bang?

If the universe began billions of years ago with the Big Bang, what preceded it? According to renowned physicist Stephen William Hawking, PhD: Nothing. However, no one can know for sure; any events that happened before the universe came into existence are without “observational consequence,” as Hawking put it, and therefore, we “may as well cut them out of the theory, and say that time began at the Big Bang.”

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This NASA Hubble Space Telescope image shows the distribution of dark matter in the center of the giant galaxy cluster Abell 1689, containing about 1,000 galaxies and trillions of stars.
Courtesy NASA/JPL-Caltech/ESA/Institute of Astrophysics of Andalusia, University of Basque Country/JHU

What is dark matter?

Everything and everyone is made up of “baryonic matter:” protons, neutrons, and electrons. Until about 30 years ago, astronomers thought that the universe followed the same rules—it was entirely baryonic matter. Over the last few decades, however, scientists have found that only 5 percent of the universe is composed of baryonic matter—and they can’t even detect the other 95 percent. Scientists believe that some 25 percent consists of dark matter—but nobody can say definitively what dark matter actually is or what it’s made of yet. Don’t miss these other science mysteries no one has figured out yet.

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This new NASA/ESA Hubble Space Telescope image shows a beautiful spiral galaxy known as PGC 54493, located in the constellation of Serpens (The Serpent). This galaxy is part of a galaxy cluster that has been studied by astronomers exploring an intriguing phenomenon known as weak gravitational lensing. This effect, caused by the uneven distribution of matter (including dark matter) throughout the Universe, has been explored via surveys such as the Hubble Medium Deep Survey. Dark matter is one of the great mysteries in cosmology. It behaves very differently from ordinary matter as it does not emit or absorb light or other forms of electromagnetic energy — hence the term "dark." Even though we cannot observe dark matter directly, we know it exists. One prominent piece of evidence for the existence of this mysterious matter is known as the "galaxy rotation problem." Galaxies rotate at such speeds and in such a way that ordinary matter alone — the stuff we see — would not be able to hold them together. The amount of mass that is "missing" visibly is dark matter, which is thought to make up some 27 percent of the total contents of the Universe, with dark energy and normal matter making up the rest. PGC 55493 has been studied in connection with an effect known as cosmic shearing. This is a weak gravitational lensing effect that creates tiny distortions in images of distant galaxies.
Courtesy NASA Goddard

What is dark energy?

The other 70 percent of the universe is thought to be comprised of dark energy. What scientists do know about dark energy is that it has a gravitationally repulsive effect (which is to say, the opposite of gravitational pull) and that it’s responsible for the expansion of the universe. But what exactly is dark energy continues to elude scientists, leaving NASA scratching their heads and wondering if Einstein got his theory of gravity wrong. One thing Albert Einstein definitely got right was the secret to happiness—and it’s genius.

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The crew of STS-45 is already training for its March 1992 mission, including stints on the KC-135 zero-gravity-simulating aircraft. Shown with an inflatable globe are, clockwise from the top, C. Michael Foale, mission specialist; Dirk Frimout, payload specialist; Brian Duffy, pilot; Charles R. (Rick) Chappell, backup payload specialist; Charles F. Bolden, mission commander; Byron K. Lichtenberg, payload specialist; and Kathryn D. Sullivan, payload commander.
Courtesy NASA

How can gravity be so strong and yet so weak?

Gravity poses a vexing paradox that can be summed up with this question: Why can a mere refrigerator magnet defy an entire planet’s gravitational pull? Some scientists believe that gravity may well be as strong as the other fundamental forces (such as electromagnetism and the force that holds the nucleus together), but its influence is dissipated because it leaks into extra dimensions. That explanation will have to do until they come up something better.

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The dwarf galaxy NGC 4214 is ablaze with young stars and gas clouds. Located around 10 million light-years away in the constellation of Canes Venatici (The Hunting Dogs), the galaxy's close proximity, combined with the wide variety of evolutionary stages among the stars, make it an ideal laboratory to research the triggers of star formation and evolution. Intricate patterns of glowing hydrogen formed during the star-birthing process, cavities blown clear of gas by stellar winds, and bright stellar clusters of NGC 4214 can be seen in this optical and near-infrared image. Observations of this dwarf galaxy have also revealed clusters of much older red supergiant stars. Additional older stars can be seen dotted all across the galaxy. The variety of stars at different stages in their evolution indicates that the recent and ongoing starburst periods are not the first, and the galaxy's abundant supply of hydrogen means that star formation will continue into the future. This color image was taken using the Wide Field Camera 3 in December 2009.
Courtesy NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration Acknowledgment: R. O'Connell (University of Virginia) and the WFC3 Scientific Oversight Committee

How big is the universe?

If scientists know that the universe is expanding, then they must be able to hazard a guess at how big it is, right? Unfortunately, scientists can’t pin a number on it. They would have to know the universe’s shape and rate of expansion, among other things—and those are mysteries as well.

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NASA's Hubble Space Telescope has helped astronomers find the final piece of a celestial puzzle by nabbing a third runaway star. As British royal families fought the War of the Roses in the 1400s for control of England's throne, a grouping of stars was waging its own contentious skirmish — a star war far away in the Orion Nebula. The stars were battling each other in a gravitational tussle, which ended with the system breaking apart and at least three stars being ejected in different directions. The speedy, wayward stars went unnoticed for hundreds of years until, over the past few decades, two of them were spotted in infrared and radio observations, which could penetrate the thick dust in the Orion Nebula.
Courtesy NASA Goddard

What is the shape of the universe?

Scientists have a number of theories about the shape of the universe. But according to NASA, the leading theory is that the universe is flat, which leads them to believe that it’s infinite in size. However, since the universe has a known beginning, and perhaps an expected endpoint, we can only observe a finite volume of it. As NASA says, “All we can truly conclude is that the universe is much larger than the volume we can directly observe.”

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This image is from NASA Galaxy Evolution Explorer is an observation of the large galaxy in Andromeda, Messier 31. The Andromeda galaxy is the most massive in the local group of galaxies that includes our Milky Way.
Courtesy NASA/JPL/California Institute of Technology

How fast is the universe expanding?

For the past six years, the Hubble Space Telescope has been calculating the speed at which the universe is expanding and recently came up with what is accepted as the most precise measurement to date—except that the rate stands in direct conflict with independent (and supposedly precise) measurements of the early universe’s expansion. That’s unsettling for astrophysicists because it means we know even less about our universe and its ways than we thought. “The community is really grappling with understanding the meaning of this discrepancy,” said the lead researcher, Adam Riess, a Nobel Laureate and Bloomberg Distinguished Professor at Johns Hopkins University. Find out the 17 scientific facts that turned out not to be true.

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Parallel universe science theory

 Is the universe actually a multiverse?

“Our universe could be just one of an infinite number of universes making up a ‘multiverse,'” according to astronomy news site space.com, and as bizarre as that may seem, there’s actual science behind it. Because there are only a finite number of ways particles can be arranged in space and time (and remember the universe is flat and infinite), then the universe would have to start repeating at some point. Which leads to the question…

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This artist concept depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone, a range of distance from a star where liquid water might pool on the planet surface.
Courtesy NASA/Ames/SETI Institute/JPL-Caltech

Could there be another you?

If the universe is compelled to repeat itself—to be part of a multiverse—could there be another version (or multiple versions) of you? “If you look far enough, you would encounter another version of you—in fact, infinite versions of you,” according to space.com. “Some of these twins will be doing exactly what you’re doing right now, while others will have worn a different sweater this morning, and still others will have made vastly different career and life choices.” This counts as one of the mysteries we’re unlikely to ever get an answer for, but it’s intriguing nonetheless. (Maybe the other version of you isn’t hopelessly addicted to Girl Scout cookies.) The experts say that this theory would require additional dimensions, and if so…

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Futuristic timetravel or space warp. 3D rendering
Fred Mantel/Shutterstock

How many dimensions are there really?

The three dimensions of which we can be certain pertain to what we can perceive: length, width, and depth. Scientists also point to the fourth dimension of time. But according to Phys.org, while still a mystery, some potential dimensions include:

  • Fifth: In which we can see a world slightly different from our own (which would give us the means of gauging similarities and differences between it and ours).
  • Sixth: We can see all possible universes that have begun the same as our own
  • Seventh: We can perceive all possible universes that began differently than our own.
  • Eighth: We see all possible universes. Period.
  • Ninth: We are aware of all possible laws of physics and universe beginnings.
  • Tenth: We can see all things possible and imaginable.

If you’re looking for more science fun, don’t miss these 18 science facts you never learned in school.

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This view of the north polar region of the Moon was obtained by NASA's Galileo camera during the spacecraft flyby of the Earth-Moon system on December 7 and 8, 1992.
Courtesy NASA/JPL

How did the moon form?

The moon’s gravitational pull is responsible for the rise and fall of the ocean tides, but how did the moon come to be? After all, it took a hundred million years after the solar system and its planets began to form before the moon actually sprang into existence. Several theories exist, none of which is close to being proven. Meanwhile, one potential key to figuring out how we got a moon is to understand why Venus doesn’t have one, according to Dave Stevenson, a professor of planetary science at the California Institute of Technology. We still haven’t figured that one out, either.

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This composite image of southern Africa and the surrounding oceans was captured by six orbits of the NASA/NOAA Suomi National Polar-orbiting Partnership spacecraft on April 9, 2015, by the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument. Tropical Cyclone Joalane can be seen over the Indian Ocean. Winds, tides and density differences constantly stir the oceans while phytoplankton continually grow and die. Orbiting radiometers such as VIIRS allows scientists to track this variability over time and contribute to better understanding of ocean processes that are beneficial to human survival on Earth.
Courtesy Ocean Biology Processing Group

Did life originate on Earth?

We take it for granted that life on Earth actually began here—but that’s not actually accurate. Scientists are finding evidence that life on Earth may have come from Mars and was brought to this planet by a meteorite. The trouble is that scientists can’t even agree on which area of science will provide the answer, let alone whether science is even where we should look.

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3D UFO over the sea and waves

Is there alien life?

Since the early 1960s, scientists have theorized that it’s extremely likely that intelligent life exists outside of Earth. The trouble is that we have zero scientific evidence. In fact, we have no hard scientific evidence that any life exists outside of earth, at least so far. But the time may be drawing closer when science confirms that there is life elsewhere in our solar system, or the galaxy, or at least somewhere in the universe.

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Sun beams or rays breaking through the dark clouds at sunset. Hope, prayer, God's mercy and grace. Beautiful spectacular conceptual meditation background.

Who’s in charge here?

Does someone or something start the whole process that led to you reading these words on an electronic screen? Or is everything that’s happened in the history of ever simply the result of a process guided by laws of science? While many notable scientific thinkers say God is a myth, not all scientists are atheists. This is one we’re unlikely to solve—at least during our lifetime. Once you get your brain back together, don’t miss these 24 astronomy facts you never learned in school.