system
Given the following post, respond as if you're commenting on Reddit:
human
What do scientists mean when they say "We only know what makes up 5% of the Universe"? What makes up the other 95% of the Universe and how come we don't know what it is ?
gpt
We can measure the total matter density.

We can also observe how many stars there are and what their masses are (and calculate the energy corresponding to this mass). They contribute 1% to the total energy density. Add gas in galaxies, gas between galaxies, planets, black holes, and everything else we can directly observe and you end up with about 20% of the total matter density.

What about the rest? We know it is there from its gravitational influence, we can even map its density, but we don't know which particles it is made out of (there are searches for possible particles, but without a discovery so far). This is dark matter, contributing about 80% of the total matter density.

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We can measure the expansion rate of space, and the total matter density allows to calculate how much expansion should slow down over time. But the measurements show it is expanding. There are multiple ways to describe this, but the easiest approach is dark energy: A constant energy density everywhere that - unlike matter - makes the universe expand faster. To compare this to the matter density, we can convert the matter density to an energy density via E=mc^(2). The overall result is 68% dark energy, 27% dark matter and 5% regular matter.