Ten things astronomy cannot yet answer.
Science is not a collection of facts. It is a collection of methods for finding out. These are the places where the methods have not yet reached. They are not failures. They are invitations.
What is dark matter?
We know dark matter exists because galaxies rotate faster than they should given the visible mass we can see. Something invisible is providing extra gravitational pull. We call it dark matter because it does not emit, reflect, or absorb light. It makes up about 27 percent of the total energy content of the universe.
We have never directly detected a dark matter particle. Experiments built to find it have kept coming back empty. The leading candidates are WIMPs and axions, but decades of searching have not confirmed either. Some physicists argue the problem might not be unknown matter at all, but a breakdown in our theory of gravity at galactic scales.
What is dark energy?
In 1998 astronomers discovered that the universe's expansion is not slowing down, it is accelerating. Something is pushing everything apart, and that push is getting stronger. We call it dark energy. It makes up roughly 68 percent of the total energy in the universe, which means most of reality is dominated by something we still cannot explain.
The simplest explanation is that empty space has energy, a cosmological constant. But when physicists calculate how much energy empty space should have, they get a number absurdly larger than what we observe. That mismatch may be the worst prediction in the history of physics. Some researchers think dark energy may vary over time rather than stay fixed.
Are we alone?
The universe contains roughly 2 trillion galaxies, each with hundreds of billions of stars, and most of those stars likely have planets. Even conservative estimates suggest billions of potentially habitable worlds in our galaxy alone. That makes the silence difficult to ignore.
In more than 60 years of searching, we have detected no confirmed signal from another civilization. Life might be extraordinarily rare. Civilizations might destroy themselves before they can communicate. Other civilizations might exist but have no interest in contact. Or we might simply be early, one of the first technological species in a universe still young enough for others to emerge later.
What happened before the Big Bang?
General relativity breaks down at the extreme densities of the Big Bang. Our standard cosmological model begins a fraction of a second after it and cannot be extended backward through it. Whether time itself had a beginning, whether the universe emerged from a previous state, and whether the laws of physics even existed beforehand are all open questions.
Some physicists propose cyclic models in which universes repeatedly collapse and expand. Others argue the Big Bang may have emerged from a quantum fluctuation. Still others think the question itself may be meaningless, like asking what lies north of the North Pole. We have no observational data from before the Big Bang, and it is not clear that we ever could.
What is inside a black hole?
General relativity predicts a singularity at the center of a black hole, a point of infinite density where the known laws of physics cease to function. Most physicists treat that not as a literal thing, but as a sign that our theory is incomplete.
A full quantum theory of gravity would likely replace the singularity with something finite and physically meaningful. But we do not yet have that theory. Whether information is preserved or destroyed, whether the interior has structure, and whether it connects to anything else remain unknown. The event horizon prevents any direct observation from outside.
Why does the universe have more matter than antimatter?
The laws of physics suggest that matter and antimatter should have been created in equal amounts at the Big Bang. If that had happened exactly, they would have annihilated each other and left behind only radiation. Instead, there is matter: galaxies, stars, planets, and us.
Something created a tiny asymmetry, about one extra particle of matter for every billion particle-antiparticle pairs. Particle physics experiments have measured some CP violation, but not enough to explain the amount of matter we see today. We exist because of an imbalance whose origin remains mysterious.
Is the universe infinite?
The observable universe is finite, bounded by how far light has traveled since the Big Bang. But the total universe might be far larger, possibly infinite. From inside it, we cannot tell. If the universe is flat, which current measurements suggest, and extends beyond what we can observe, infinity becomes a real possibility.
An infinite universe would have strange consequences. In an infinite space with finite kinds of physical arrangements, every possible arrangement of matter should eventually repeat. That implies copies, recurrences, and scales that most physicists find deeply uncomfortable, even if they cannot rule them out.
How did the first stars form?
The very first stars, sometimes called Population III stars, formed from the hydrogen and helium created in the Big Bang. No heavier elements existed yet. These stars were probably enormous, bright, and short-lived, and they likely forged the first heavy elements when they exploded.
We have never directly observed a Population III star. They formed too early and too far away. The James Webb Space Telescope is searching for indirect evidence of them in the earliest galaxies, but for now the first stars remain an inference rather than a sighting.
Will we ever travel to another star?
Proxima Centauri, the nearest star, is 4.2 light-years away. At Voyager 1's speed, the journey would take about 70,000 years. Reaching another star in a human lifetime would require traveling at a substantial fraction of the speed of light.
No known physics forbids that in principle. Ideas such as laser-pushed light sails, nuclear pulse propulsion, and antimatter engines have all been proposed. But the engineering gap between where we are now and what such a mission would demand is enormous. The limiting factor may not be physics so much as long-term human choice.
What is the ultimate fate of the universe?
If dark energy keeps accelerating the expansion of the universe at its current rate, the long-term outcome is the Big Freeze, or Heat Death. Over trillions of years, distant galaxies disappear beyond the observable horizon, stars burn out, and black holes eventually evaporate through Hawking radiation.
That would leave behind a cold, dark, nearly featureless cosmos at maximum entropy. Other possibilities, such as a Big Rip or a Big Crunch, seem less likely but are not entirely ruled out. We are also not certain the current laws of physics remain stable forever. Even the vacuum state of the universe may not be permanent.
Not knowing is not the same as not understanding.
Every question on this page is the product of centuries of careful observation, mathematical reasoning, and instrument building. We do not know the answers. We know, with great precision, where the answers are not. That is science at its most honest and its most interesting.
Read what we do know →