If you really want to overload your mind thinking about this, imagine this universe is only a bubble crowded into a group of other bubbles, like a kid blowing soap bubbles.
So the pressure around our bubble is not uniform, there are more bubbles on one side than another, other bubbles are much larger and some are very tiny causing tiny "lumps" of pressure in various places on our bubble.
Decades ago I really liked the "big collapse" theory that has now been abandoned, it was so "simple" in comparison to a universe that keeps expanding and not uniformly at that.
So the pressure around our bubble is not uniform, there are more bubbles on one side than another, other bubbles are much larger and some are very tiny causing tiny "lumps" of pressure in various places on our bubble.
Decades ago I really liked the "big collapse" theory that has now been abandoned, it was so "simple" in comparison to a universe that keeps expanding and not uniformly at that.