Is the universe flat, or curved?
In Middle Ages they thought the world was flat. Is the universe flat, or does it just go on and on and on, or is it a certain shape?
The answer is that we just don't know, but what we do know for sure is that the universe is getting a lot larger. If you look at distant galaxies a long way from us, they are something called red shifted. What that means is the light waves coming from them to us have been stretched out. The reason they've been stretched out is because the space separating us and that distant galaxy has got bigger. So it stretched the waves and makes them more red-dominated than it should be. That's a similar reason that when a police car comes towards you, it sounds different for when it's going away from you because of something called the Doppler shift.