For the purpose of this questions let's assume that the physics of our universe can be fully described by a complete non-contradictory theory (i.e. that theory of everything exists). Then our universe can be described by that theory and an initial state, i.e. a pair Universe0 = (ToE0, S00) in a sense that (assuming for simplicity the physics of our universe can be described by an evolution of a state in discrete or continuous time) any future state can (in math sense) be uniquely determined from S00 by applying the laws ToE0. Imagine some other non-contradictory theory and an initial state Universe1 = (ToE1, S10). E.g. a huge deterministic virtual machine with discrete time, or a universe with more familiar spacetime, but with 5+1 instead of 3+1 dimensions. For simplicity assume that no full simulations of Universe1 were ran in Universe0 and vice versa.
All of us, including all the readers of that question on StackExchange are parts of Universe0, and the neurons in our brains react to this question exactly as follows from ToE0 and the initial state. Most of us believe that we exist.
If the laws of ToE1 and the initial state S10 lead to complex structures being part of Universe1 being able to wonder about its fundamental laws and maybe even ask each other questions similar to this one, then such structures might believe that they exist for the same reason we do (why wouldn't they?). In other words it appears that Universe1 can be as subjectively real for such structures as Universe0 is subjectively real for us.
It appears that the situation is symmetric. Universe0 for structures in Universe1 is no more than Universe1 for humans in our universe (Universe0): in both cases the other universe is just a non-contradicting theory describing its evolution and the initial state. Does that mean that Universe1 actually exists in the same sense our universe exists? Or is there something which makes us and our universe more objectively physically real than Universe1 or any other theoretically possible universe? And if there is no such objective difference, isn't that the answer to "Why is there something instead of nothing?"?