As I understand it, the jury is still out. A lot of people have done a lot of math about it, but we still lack some empirical data to check that math against. There's a number of possible options that are consistent with current observations.

> The idea that space is infinite makes no sense to me as even the physical “space” between planets, stars etc is still “something” and according to the Law of Conservation of Mass, matter cannot be created or destroyed.

That space is, I suppose, "something" in a sense, but it certainly isn't matter. It's just location. (And matter can be created and destroyed; it's just converted to or from energy when that happens. This is what E=mc^2 is about. But the expansion of the universe is creating neither matter nor energy - just locations for it to be in.)

> BUT if space is finite, that suggests if you were able to travel for sufficient time, you would eventually reach the end/edge, which surely suggests it is contained within something. If this is true what/who is outside “the box”?

Not necessarily. The two-dimensional surface of a three-dimensional sphere has no edge. Our three-dimensional universe could be arranged in some four-dimensional way such that it has no edge.

In any case, since it's impossible to observe what's outside the universe (by definition of 'the universe'), science can simply say nothing at all about the outside of it.