How can something infinite be expanding? What was there before it expanded into it?
I think that is one of the big questions.
Who said the universe is infinite? How do they know?
The universe is expands the space between objects like two points on the surface of a ballon as it is filled with gas; however there is no way to know if the universe its self is expanding as we cannot see the (present) boundaries (if any) in real time. We cannot be sure that the universe is not a set size with objects rushing apart to fill the unobserved (and perhaps unobservable) boundaries ...
Objects get further apart, but conversely all objects are going to the same place (at varying speeds) - called the Great Attractor.
Recommend some form of aspirin at this point for incipient headaches brought on by attempting to make sense of something that by definition does not make sense except in mathematics ...