The expansion of space.
What is the space? When you look at what looks like nothing, it's nothing, it's space. Is empty space something? The laws of the universe are described in empty space, which is not empty.
Everything we see is basically variations of empty space. A particle is merely an energetic rise in a specific location in the matrix of space. And when we don't perceive anything in space, we still have the grid with its positions for specific energy variations.
When we look at the universe, we see that it is expanding, but what would it have outside of the universe?
If there was something outside there would be pressure or for the limits of the universe to slow down or expand. But what you have outside is nothing, and when you say nothing, you don't even have the matrix that manages all the energy so that everything is what it is.
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https://www.youtube.com/watch?v=RTLlL9BWzWg