Singularities
We can't know because no light can escape it.
What is inside a black hole?
It will most likely keep coasting along, expanding and cooling.
What will happen to the Universe?
We are developing theories rapidly as we get new data from JWST.
Why do some galaxies look weird?
We probably don't understand it well in extreme conditions.
What's up with gravity?
We don't have a good way to estimate a probability.
What is the probability of (intelligent) life?
We believe that the black holes inject energy into the surrounding gas, preventing star formation.
What is the relationship between AGN and their host galaxies/clusters?
There's about a 50% chance.
Will we collide with Andromeda?
We know about some big ones like the Gaia Sausage.
How many merger events built the Milky Way?
It's probably NOT a bunch of yet-unobserved planets and/or brown dwarfs.
What is dark matter?
We have observed a large handful, but it requires very detailed measurements of the atmosphere and composition.
What fraction of exoplanets are habitable?
Either they formed there directly or they started out small and grew very quickly.
How did we get SMBHs in the center of (nearly) every galaxy?
We can't agree on whether its 67 or 73 km/s/Mpc.
How fast is the Universe expanding?
We know that it was very early on, shortly after the first stars formed.
How did galaxies form?
It does not get diluted as the Universe expands.
What is dark energy?
We think they come from magnetized, rapidly spinning neutron stars.
What produces fast radio bursts?
It seems to disappear, which is a paradox.
What happens to information that disappears inside a black hole?
Our theories break down under the extreme physical conditions.
What happened in the very early Universe?
We think they are regions of increased stellar density.
How do galaxy spiral arms form?
We think that it is on large scales.
Is the Universe the same everywhere?
We need to measure its mass and radius to find out.
What is the composition of a neutron star?
We mostly observe them at low and high masses.
Are there intermediate mass black holes?
We think it was sometime between 400 million and 1 billion years after the Big Bang.
When did reionization happen?
It's not clear whether it grew after it got there or before.
How did we end up with a massive BCG at the center of every galaxy cluster?
We can only see part of it.
How big is the Universe?
They would violate fundamental laws.
Are there wormholes?