If so, do we know where is the middle of the universe?
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Yes. As far as we know there is no center. There could be a center but well look into why that isn’t required and isn’t the case.
So lets look at a universe with a center where every point races away from the center with a constant velocity. If we sit into one of the points that isn’t the center we can calculate what velocities we see for other points. Classically you just subtract velocities. I look at a point and subtract my velocity and if you draw out a few points and just look at the resulting vectors (assuming the subtraction of geometric vectors isn’t new) you’ll see that looking from either point it looks like it’s the center.
So for us to look like we are in the middle is consistent with many models. The actual expansion of the universe has no center and in that case the geometric idea we discussed also applies. Everything races away from everything. This is accelerating and is proportional to distance. Think of it like space is stretching like a rubber band. Points near to each other move apart slowly while far away points have a lot of rubber between them to stretch so they drift apart quickly.
Or you can think like space gets added, like they are cells tha multiply. The more space you have between to point the more space multiplies.
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