Dark matter is the vast majority of the mass
of the entire universe.
It's the mass that holds all galaxies together,
and in fact, led to the formation of galaxies.
And it also holds clusters together and it
made the most important contribution to the
organization of the structure of the universe.
We already know that the dark matter is cold.
I invented this terminology back in 1983,
calling the dark matter hot, warm or cold
depending on how rapidly it's moving in the
early stages of the Big Bang.
Hot if it's moving at nearly the speed of
light, cold if it's moving so slowly that
its motion hardly matters, and warm is an
intermediate case.
We know that the dark matter has to be pretty
cold, but it could be a little bit warm.
And that would make a great difference to
what we call small scale structure, the amount
of satellite galaxies and things like that.
We don't yet know the real nature of the dark
matter beyond that it's pretty cold.
Being pretty cold is enough to allow us to
predict in great detail the large scale structure
of the universe, the organization of the galaxies
and to some extent the satellites of the galaxies.
But the small scale structure of the universe
really depends in more detail of the nature
of the dark matter.
Also, the dark matter can possibly interact
with itself and annihilate and two dark matter
particles come together and then make a lot
of other stuff.
And this could have played an extremely important
role in the early universe, and it could still
be producing effects that are sensitive detectors
in space and on the ground can find experimentally.
We haven't yet seen clear evidence for any
of these things, although there are a number
of experiments that are reporting tentative
detections.
So, it feels very much like we're on the verge
of major breakthroughs in trying to understand
the nature of the dark matter.
If we finally do figure out the nature of
the dark matter, we will then have a single
unified picture of the origin and evolution
of the entire universe.
One that scientists all over the world have
contributed to and that can become the basis
for a shared origin story that could possibly
solidify the bonds of humankind.
We've never had a single picture, thoroughly
supported by scientific evidence, and we're
coming close to it now.
So I think we scientists are feeling very
hopeful that we're about to cross this threshold
and have a complete understanding of the origin
and the evolution of the universe.
And of course, we're also coming to a much
better understanding of the evolution of life.
So these last decades of the 20th century
and the first decades of the 21st century
are a real turning point, I think, in our
understanding of how we got here.
