My question is, If space is a vacuum, how can that happen without being encased inside something? So could we be inside something we can't see or detect?
It rather depends on how you conceive the situation. Space is indeed inside something - that thing being the "universe", which contains everything.
And because the universe encompasses all things by definition, there is no need for the universe itself to be contained (because there exists no outer area into which anything inside the universe can move, or into which the universe can expand).
Also, the existence of space is, broadly speaking, the counterpart to the objects in it. That is, you cannot have objects without having space between them (because objects without space between them cannot sustain any division between each other, and a single object simply becomes equivalent to "the whole universe"), and you cannot have space without any objects in it (because that simply becomes "nothing"). Space itself sometimes takes on the form of an object in physics, to which properties are ascribed (i.e. it's not truly treated as "empty space", but as an object in itself which fills the area between all other non-space objects, as water fills the gaps between fish).
Objects and space are, to a certain extent, self-referential in their definitions (with each defined in terms of being what the other is not), and they form a set of basic axioms about how we see the world. At this level the question ceases to be a physical one, but a very general philosophical one, and is not something that a physicist or any natural scientist would (in that capacity) be in a position to address or contribute any expertise.