[D Rider]

It is a mathematical concept, and is not limited to a line. One of the outcomes of the theories of relativity is the finite universe that has a beginning (big bang) and will have an end (heat death.) Quantum mechanics also defines the finiteness on the micro scale, both time and length have limits of smallness.
While the universe is very large, it has limits.

[Ouderkirk]

If the universe is finite, then what does it end to? One assumes the big bang occurred at a single point, and the expansion reaches a certain limit and then contracts...what is beyond the limits of that expansion? Is it then the mobius strip theory of space?

[UCANSEE2]

While the universe is very large, it has limits.

So far, the only known limit to the Universe is the Speed Limit.