On the other hand, if the dark matter is very,

very light, then it wouldn't actually fit inside our own galaxy.

And if, you may know, that electron is

orbiting around the nucleus, so in that case,

electron has sort of a shape and certain

radius, and that determines the size of an atom.

In the same way if the matter is rotating

in the gravitational field coming out from a given galaxy,

they would also give you a blob and shape of

dark matter which has a characteristic size or Bohr radius.

So, you can work this out, using the law of Physics called Quantum Mechanics.

And if you're not familiar with this, don't worry about it.

So, you can compute given a gravitational force depending on the mass of a

[UNKNOWN]

galaxy. And little m is the mass of dark matter.

This is the size of the orbit of the

dark matter in this gravitation field and that goes.

Up as inverse squared of the mass.

So the lighter the dark matter is, it becomes fluffy and wider spread.

So at some point it wouldn't fit in a galaxy at all.

So because dark matter has to fit inside the size of a galaxy

the mass cannot be smaller than something.

And that number turns out to be actually this one here.

10 to the minus 22 electron volt if you look at the mc squared.

So that gives you a range.

So we know that it has to be less than ten

to minus seven solar masses because of this search for the gravittaion

[UNKNOWN].

On the other hand it has to be bigger than something,

so that the orbit of dark matter can fit inside the galaxy.

And this is what we find.

So in terms of the unit called Gigaelectron volt, has

to be somewhere between ten to the minus 31 and ten

to the 50th, so we manage to limit the range

of dark matter mass within eight one, 81 orders of magnitude.

And that's the progress we made in 70 years since Mister Zwicky

pointed out the existence of dark matter in the 1930s.

So, the point here is that we actually know so little about the nature of

dark matter, we have pretty much verbal idea

on the mass of the dark matter individually.