Question: why is the universe expsnding

Alexander Burke answered on 14 Mar 2018:

The universe is expanding due to the presence of dark energy in the universe. Truth be told, no one really knows what dark matter/energy is. It’s a strange form of “stuff” that we use to explain strange things that happen in our universe. By an addition of a small positive constant (called a cosmological constant) in Einstein’s equations, Friedmann and other clever people were able to show that the universe must be expanding providing that there is matter (heavy stuff), radiation (light) and dark energy (??) in the universe. Edwin Hubble the first person to prove (by directly observing through telescopes), that the universe was expanding. He managed to spot that more distant objects were travelling away faster than closer ones… this was groundbreaking for the scientific community =).