The universe is expanding. 
just like a balloon expands when you put air in it. 

the speed at which the universe is expanding is hubble's constant. 
which at this point in time is i think 71kms/mpc. which means for every megaparsec [3.2 million light years], the universe is expanding at 71 km a second. 

oh ****, ive totally forgotten how to explain this myself.