mindblowingscience:

We know the age of the universe, based on the cosmic microwave background from the big bang. We don’t know whether anything really existed before that, because at the same of the formation of the universe, visible light did not exist, so we can’t really see back in time further than that.

Expansion is caused by an expansive force called Dark Energy. We do not have a clear answer what this is, but physicists have competing theories.

We know its expanding by observing the light of distant galaxies and stars shifting to the red frequencies of light, which indicates that it is moving away from us. As distance increases a red shift indicates the frequency is decreasing.

If I’m wrong about any of this, please correct me. I’m a Chemist, but Astronomy is a strong interest of mine.