Take a saucepan or like a pressure cooker that you can seal.
Just for the hey of it, make it a clear one you can see through. Fill it halfway with water.
Put it on the stove. It will start to boil, but all the “air” above it will still seem clear. It is normal air but with alot of water vapor mixed in, under huge pressure.
Now release the spigot.
You will see a huge cloud of steam emerge. stuff which seems solid but is really just stuff that condensed out of something way more dense.
That’s what mass is. We are the “steam” of the big bang. And the expansion will continue forever. We can never gather up a;; the vapor and compress it back down, realize we are not talking truly about water vapor, we are talking the fundamental stuff that matter is made of.
I came up with equations in the early 90’s that predicted:
The mass of the universe is increasing linearly with the time
The volume of the universe is increasing in a cubic relation to the time
So
All we know is getting bigger, but really evaporating before our eyes
And the numbers I was working with at the time predicted that the “age” (if you can define it that way) of the universe was 18 billion years, with an average density of about one proton mass per 10,000 cubic centimeters.
But I might be wrong.
Ok...I get that part...but doesn’t that presuppose a static amount of mass?
Incidentally, I've always been puzzled by Einstein's apparent assertion that gravity was not really a force, because modern physicists treat it as a force and look for its "force carrier" particle, the graviton.