Avi Loeb

You can still find those objects.

Right.

Among the matter, there is also dark energy on top of that.

So I was saying out of all the matter in the universe, only 16% is ordinary matter that we are made of.

So the thing about ordinary matter is that we can see it, right?

It interacts with light.

And this substance that is called dark matter, just to express our ignorance, is dark in the sense that it doesn't interact with light.

And how do we know that it exists?

It's because the amount of mass associated with big objects like galaxies or clusters of galaxies is much bigger than we can see in stars or in gas, the ordinary matter that

that we are used to and moreover we can follow the history of the universe and based on that history which we measured to exquisite precision using the cosmic microwave background we can tell that there is a substance that was not coupled

to the cosmic radiation you know this relic radiation from the hot big bang that the universe started hot and dense and then it expanded so we see the radiation left over from that hot dense phase it's called the cosmic micro background or cosmic radiation background and that radiation background was coupled to ordinary matter early on when the universe was dense and then

We would not exist if there was only ordinary matter in the universe because the ordinary matter was coupled to the radiation.

The radiation was smoothing all inhomogeneities in the ordinary matter.

So the ordinary matter was smooth just like the radiation was.

However, the dark matter was more clumped as a result of some...

physics in the early universe, that there were some inhomogeneities created in it, but it wasn't coupled to the radiation.

So there was no smoothing force acting on it.

And so the reason that the Milky Way galaxy exists is because the dark matter maintained memory of the initial inhomogeneities.

And so it collapsed.

So this region that was denser than average in the universe, mostly full of dark matter,