When dealing with infinite space of the universe, most of us could get confuses easily.

The curved surface of the Earth can be used as an analogy. The Earth's surface is finite in area, but unbounded (no boundary). A traveller will not meet any edge or boundary in whatever direction he goes.

Similarly space could be finite in volume, but without any edge or boundary. An astronaut could, in principle, pointing his rocket in one direction and would return to his starting point at the end.

The big bang theory suggests that all matters (planets, asteroids, galaxies) are moving apart from each others a.k.a expending universe.

While the matters are moving away from each others, so DOES the SPACES within them are expending (imagine the spaces are stretch in an expending universe). The matters is NOT expending in spaces that already there, but the SPACES are being stretch further when the matters expend.

Many of us often assume that universe space is infinite and the planets are moving away and filling up more spaces which is already there (like a boy moving two balls apart in a big room (where spaces already be there) which is NOT TRUE.

Universe is a hypersphere with finite volume without boundary. The volume (x, y,z, t) continues to expend (and one day may retract) as a result of the big bang explosion.

If you are interested to understand this more, please refer to Einstein's hyperspherical cosmos modeling.

Cheers.