Eventually, they touch and merge and become a single mass, which gets denser and denser. Planets and stars lose their boundaries. Atoms get ripped apart and crushed together. Everything gets squeezed closer and closer together. Finally, there comes a definite time in the past when all the matter of the universe is compressed into a single point.
Astronomers can estimate that time by measuring how fast the universe is expanding now.
It's about ten billion years ago. Ten billion years ago, according to the theory, the universe exploded from a point and was born. Scientists call that beginning the Big Bang. The old gentleman was busily working his moustache again. Furthermore, he had abandoned the safety of his chair and was pacing the room, narrowly missing the logs piled by the fireplace.