If you want to understand the Universe — and we do — you have to understand what's in it. I don't mean stars and planets and black holes and such. We need to be even broader.
How much energy is there in the Universe? How much matter? And to be a little more specific, what kinds of energy and matter?
We call this the mass/energy budget of the Universe. Like a household budget, it (hopefully) accounts for everything in it, divided by type. In the case of the Universe, we know it's made up of — in decreasing order — dark energy, dark matter, and normal matter. But how much of each?
A new study looked at just matter, and came up with a fairly narrow number: 31.5 ±1.3% of the Universe is made of matter (which, in turn, implies 68.5% is dark energy).
These numbers are pretty important. If the Universe had less matter, it would expand more rapidly — in a sense, the gravity of that matter slows the expansion.
The mass/energy budget of the Universe shows us that most of the stuff in the cosmos is dark energy, then next is dark matter, then finally the normal ...