Wdym? Do you care to elaborate?
If we can't perceive the fourth dimension, then it is not in our plane of reality. I do not believe our plane of reality has higher dimensional beings (and Flatland predates Carl Sagan by many years). I have no clue whether there are four dimensional beings or not.
The 9 additional dimensions in 11th dimensional super stringer theory are hypothesized to be unseeable quantum dimensions. We do not know if these exist.
According to Einstein, gravity is the warping of space-time (time is the fourth dimension in general relativity)
I prefer to think of it as the softening of space-time. I have to have one more spatial dimension than the thing that is being warped, and picturing a fourth spatial dimension for me is mathematical rather than visual.
In contrast, with softening I can picture a plexiglass cube under pressure, where mass/energy creates soft spots in it. No extra dimensions are required, the softening naturally spreads with the inverse square of the distance, and the speed of a wave (or time) in a soft area will naturally be slower, and things will naturally migrate to the software areas, so gravity happens naturally as well. Also, it provides a natural explanation for an event horizon – it is the horizon at which the stiffness reaches zero.
I do not know if this is distinguishable mathematically from warping (I have not yet found any mathematical difference), but it is much easier for me to visualize.
I suspect that it is also connected to string theory because the softness could also be kinks into additional dimensions, with the hidden dimensionscorresponding to the other fundamental forces.
(Another reason that I personally prefer it is that I came up with it myself, but that probably doesn't matter too much to the universe as a whole).
As far as I know, the universe is not expanding into anything, it is expanding into nothing.
You can call areas outside of the observable universe (our bubble) a parallel universe since it would never be reachable, or so we now think (current space-time laws). This is one version of the multiverse. If the universe is infinite then you have these areas that you're never going to reach so thats why in this sense a multiverse. We do not know if the local universe is truly infinite
The speed of light is through space-time so we don't know if there's stuff beyond space-time. But in theoretical physics there is. The brane and the bulk so there's other actual potential physical dimensions. Things that connect blackholes, whats outside of things connecting them - That is often seen as the lowest level of the multiverse and parallel universes.
I suspect that the multiverse includes universes with different laws as well as extensions of our universe.
en.wikipedia.org