A way of thinking about it is that space isn't "something". Space is just the gaps between "something".
It seems like a lot of people think of the universe as a sort of container/volume that things exist inside, and then what is "outside the box" is a natural question, because there is a boundary to pass.
If you instead think of it as simply a description of the extent of a set of objects, it's easier. Picture a simulated universe with no fixed boundary, just a set of objects with coordinates.
"The universe" is just the set of objects, and it's volume is just the volume encompassing all of those objects at any time. The simulation doesn't contain any "space". Space in the simulation is just an absence of objects, and there's nothing special about "outside" or "inside" in that situation. If the objects move further away, the universe gets bigger. If you "travel" to the edge of the universe, and travel further, the universe gets bigger. If you try to traverse the current "edge of the universe", the edge moves with you.
There are theories where there might be something "outside" our universe in one or more dimensions, but there's no reason why something absolutely needs to have an "outside".
Does that include photons? Photons that have travelled the furthest are, by the above definition, at the boundary of the universe - an ever expanding boundary - which they will never exceed. If this is so and you were 'outside' the universe, would it look like a black hole? Does a universe have an event horizon?
It seems like a lot of people think of the universe as a sort of container/volume that things exist inside, and then what is "outside the box" is a natural question, because there is a boundary to pass.
If you instead think of it as simply a description of the extent of a set of objects, it's easier. Picture a simulated universe with no fixed boundary, just a set of objects with coordinates.
"The universe" is just the set of objects, and it's volume is just the volume encompassing all of those objects at any time. The simulation doesn't contain any "space". Space in the simulation is just an absence of objects, and there's nothing special about "outside" or "inside" in that situation. If the objects move further away, the universe gets bigger. If you "travel" to the edge of the universe, and travel further, the universe gets bigger. If you try to traverse the current "edge of the universe", the edge moves with you.
There are theories where there might be something "outside" our universe in one or more dimensions, but there's no reason why something absolutely needs to have an "outside".