For six decades, NASA has been exploring the mysteries of the universe, from those hiding in our own solar system to the very origins of time and space. But for every answer found, new questions emerge, and some have proven tougher to crack than others. Here are some of the most persistent mysteries that even NASA can’t explain—yet.
1. Green spark in Fireworks Galaxy
A surprise “green blob” was recently captured lighting up the Fireworks Galaxy, puzzling astronomers with its rapid appearance and disappearance. It emitted ultraluminous X-rays but no visible light, ruling out a supernova. One possible explanation for the source of the ephemeral burst is a black hole tearing apart a small star. Another theory suggests it came from a neutron star, although the rapid spinning of neutron stars creates a fairly impenetrable magnetic barrier. How the material might have gotten through remains a mystery.
2. The Great Attractor
There’s something irresistible buried in the middle of our local supercluster that’s drawing galaxies in like moths to a flame: a gravitational anomaly known as the Great Attractor. Our Milky Way is hurtling towards it at a dizzying rate of 630 kilometers (391 miles) per second. This speed can’t be fully explained by the supercluster’s mass, nor does it correspond with the repulsive effect of a nearby void in space. The disc of our own galaxy may also be obscuring what exactly is pulling us in.
3. Mass in the Moon
A mysterious metallic mass has been discovered on the dark side of the Moon. Researchers describe the underground “blob,” which extends 300 kilometers (186 miles) deep, as “a pile of metal five times the size of the Big Island of Hawaii.” The aim of future missions will be to unlock the secret of how it formed, whether from an asteroid impact or remnants of ancient oceans of magma when the Moon was still young.
4. Lunar water
Nearly 40 years after lunar rocks were brought back to Earth by the Apollo mission, scientists were surprised to discover they contained water. Since then, the presence of water has been confirmed in pretty much all areas of the Moon, even in the form of surface ice. However, its origins are still a big mystery: theories include “seeding” from meteor and asteroid impacts and hydrogen from solar wind reacting with oxygen in the moon’s regolith.
5. Cold spot
There’s an unexplained cold spot in the cosmic microwave background, the faint radiation left over from the Big Bang itself. One study posits that the cause is a supervoid—a vast expanse of space with less matter than usual—1.8 billion light-years across. (A smaller void may even surround our very own Milky Way.) Other astronomers disagree, speculating the cold spot might be due to an aggregation of smaller voids, or more excitingly, a collision with another bubble universe.
6. Dark matter
Dark matter is one of the “biggest questions in science.” It doesn’t interact with light or ordinary matter, and yet everything behaves as if it’s there; without the mass attributed to dark matter, galaxies would fly apart. There’s also a lot of it—dark matter accounts for 85 percent of all the matter in the universe. Theories on what it’s made of range from primordial black holes to “hairy” filaments to exotic particles such as sterile neutrinos.
7. Missing matter
Normal matter, a.k.a. the baryonic matter has been taking a bit of a beating in cosmology. Not only has it been found to make up a mere five percent of the universe (the rest being dark matter and dark energy), but a third of it is missing, which presents one of the “biggest conundrums in astrophysics.” Thanks to the Chandra X-ray Observatory, astronomers now think it might be hiding in the warm-hot intergalactic medium (WHIM), long filaments of gas in the space between the stars.
8. Quintessential dark energy
Something is driving our universe to expand ever faster when gravity should be pulling it inward. This unseen, unknown force is known as dark energy, or “quintessence.” It seems to get stronger as it goes and accounts for more than two-thirds of everything in the universe. Unfortunately, that’s about as much as we know. The alternative—Einstein’s theory of gravity is wrong and we need a new one—isn’t much better. NASA’s new WFIRST, or Wide Field Infrared Survey Telescope, should help shed some light on the matter.
Our first known interstellar visitor was an oblong rock christened ‘Oumuamua (Hawaiian for “a messenger from afar arriving first”) that defied what we know about comets and asteroids, leading some to speculate it might be an alien probe. Unusually elongated, made of rock and possibly metal, and containing no water or ice, it bore signs of having spent hundreds of millions of years out in space. It was too small and too fast to capture in an image, but scientists hope it is but the first of many such messengers from afar.
10. Planet Nine
Our solar system may have lost Pluto as a planet, but astronomers say there’s still a ninth one out on the fringes just waiting to be found. (Ironically, one of those astronomers was also instrumental in Pluto’s demotion.) Planet Nine’s existence was suggested by the bizarre orbits of objects in the Kuiper Belt, and simulations suggest it’s huge—ten times the mass of Earth—taking up to 20,000 years to orbit the sun.
11. Fast radio bursts from Space
In 2007, astronomers discovered that the Earth is periodically blasted with ultra-powerful radio waves that appear all over the sky for a few milliseconds before they vanish. These fast radio bursts are relatively rare, making it hard to pinpoint their origins. A few “repeaters” have been observed; some, curiously, emit signals with a descending frequency, which researchers have likened to the sound of a “sad trombone.” This cosmic puzzle has generated at least 48 different theories, but a new Canadian telescope may soon whittle those down.
12. Black hole paradox
Once deemed impossible, black holes have been found at the heart of most galaxies, and we’ve even managed to photograph one. These objects are incredibly dense and possess a gravity so strong that nothing that falls in can escape. But this creates a paradox: if black holes can completely evaporate, as Stephen Hawking showed, what happens to the information about what went on inside? As far as we know, information can’t be destroyed. There are many proposed ways a black hole could “leak” information, but none fully jibe with our understanding of physics.
13. White holes
Black holes have long held our fascination as singularities from which no information can emerge and hat are unknowable by nature. But what about white holes? Einstein’s theory of general relativity predicts this mirror image must also exist, belching out matter the way its darker cousin consumes it. Various theories suggest that white holes and black holes are connected via wormholes, that white holes are the ultimate fate of black holes, and that dark matter may be made up of primordial white holes.
14. A very fickle constant
A constant, as the name implies, should be a fixed quantity, but the Hubble constant simply refuses to stay put. A century ago, Edwin Hubble discovered our universe was expanding; since then, astronomers have pinned very precise, very different values on the rate of that expansion. If these discrepancies can’t be resolved, scientists may need an entirely “new model for the underlying physics of the universe.”
15. The fate of the universe
Will the universe go out with a whimper or a bang? For decades, the answer to this question hinged on how much matter was in the universe. Was there enough to override the momentum of expansion and pull everything inward in a Big Crunch, or would expansion win, causing the universe to peter out into a Big Freeze? The discovery of dark energy favors endless expansion unless runaway dark energy ends up tearing the universe apart—the Big Rip.