eli5: Does expansion of the universe not cause reduction in mass and energy? How does does the mass and energy of an expanding universe remain constant and not reduce or diffuse?
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There are many things about the universe we don’t understand. Mass and energy are two examples. With mass, when we measure the mass of everything visible, gravity shouldn’t work the way it does. We can only see 15% of the mass needed in the visible universe for things to move the way they do based on gravity. That other 85% of the mass that must be there and we just can’t see is called “dark matter” because our measurements and understanding of the universe say it must be there, but we can’t see it.
Then there’s the energy aspect. Again like you said for the universe to keep expanding, there must be energy responsible for that. But we can’t measure that energy. Now we know from Einstein that energy and mass are related and can be converted between the two, so when we add all of the mass and energy we can identify, even including all of that dark matter, almost 70% of the energy needed for the universe to expand the way we see it can’t be understood or identified. We call this “dark energy”.
Putting dark matter and dark energy together, it accounts for 95% of the observable universe. Let me say that again, 95% of the observable universe is completely unknown what it is or how it works, but based on the math and physics we do understand, must be there.
Tl;dr we don’t know, and very likely will never know, almost certainly not in our lifetimes.
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