In 1929 the astronomer Edwin Hubble measured the velocities of a large selection of galaxies. He expected that about equal numbers would be moving toward and away from us. After all, the Earth isn't a particularly special place in the universe. Instead, he discovered that almost all galaxies are moving away from us! Also,Since the time of Hubble we have observed millions of galaxies with better equipment and verified his results. With the exception of a small handful of galaxies close to us, every galaxy is moving away from us.
Universe is still expanding and its very big and we only know 4percent of it and rest 96% is still unknown for example ;take a balloon & sprinkle a few drops of paint & expand the balloon what will happen? that is what happening to the universe