How can the universe be constantly expanding if the amount of matter never changes?
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Better to think of it as inflating like a balloon. The mass of the surface of the balloon didn’t increase either even though the balloon is twice its size in surface area if you inflate it.
You have to imagine the surface of the balloon is representing the universe and not the whole volume of air inside the balloon for this analogy to be a reasonable example.
The balloon analogy isn’t perfect but its a commonly used example.
Because it’s not about the amount of matter. It’s about the space between the places where matter can be.
How do I put this? Imagine a square grid, about 4cm on a side made of rubber, except the edges which are 4 separate pieces of steel with a loop sticking out at the center of each piece of steel. Every spot on the grid is filled with dice (1 per square). Tie 1 rope to each piece of steel. Give each rope to a friend. Have the friends start pulling the ropes. Harder and harder, but slowly. Pretty soon the grid is now 5cm square, then 6, then 7. The grid itself has gotten bigger. But it’s still only filled with 1 dice per square. That’s what happening with the space. The grid represents space itself, while the dice represent the matter in space. And the four friends represent dark energy (or whatever the force driving the expansion of the universe actually is – we *think* its dark energy, but we’re not 100% sure on that point).
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