The question isn't about whether dark matter
exists or not. What's going on is, when we measure gravity
in the universe—the collective gravity of the stars, the planets, the moons, the gas
clouds, the black holes, whole galaxies—when we do this, 85 percent has no known origin. So it's not a matter of whether dark matter
exists or not, it's a measurement—period. Now, “dark matter” is not even what we
should be calling it because that implies that it is matter; it implies we know something
about it that we actually don't. So a more precise labeling for it would be
“dark gravity.” Now, if I called it dark gravity, are you
going to say: “Does dark gravity really exist?” I'd say yeah, because 85 percent of the gravity
has no known origin. There it is. Let's figure out what's causing it. The fact that the word “matter” got into
that word is forcing people to say, “Well I have another idea, I bet it's not matter,
it could be something else!” We're overreacting to a label that overstates
our actual insights or knowledge into what it is we're describing. Then I just joke we should just call it Fred. Fred or Wilma, something where there is no
reference to what we think it is because in fact we have no idea. So here's how you actually measure the stuff. In a galaxy, which is the smallest aggregation
of matter where dark matter manifests, you look how fast it's rotating and we know from
laws of gravity first laid down by Johannes Kepler and then enhanced and given further
detail and deeper understanding by Isaac Newton, you write down these equations and say, look
how fast it's rotating, you invoke that rotation rate in the equation and out the other side
says how much gravity, how much mass should be there attracting you. And the more mass that's there the faster
we expect you to be orbiting. That kind of makes sense. So when you do this calculation on a galactic
scale, we get vastly more mass attracting you than we actually can detect. I'm adding up stars, gas clouds, moon, planets,
black holes—add it all up. It's a fraction of what we know is attracting
you in this orbit, and we cannot detect the rest and so we hand it this title dark matter. Understandably, I suppose. But it implies that we know that it's matter,
but we don't. We know we can't detect it in any known way
and we know it has gravity, so it really should be called dark gravity. I think the over–under on what dark matter
might be, today, I think we're all kind of leaning towards a family of particles, subatomic
particles, that have hardly any ability to interact with the particles we have come to
know and love, "ordinary matter". And that would make it matter, dark matter,
as we've all been describing it. And it's not a weird thing that you could
have a particle that doesn't interact with our particles. Within our own family of particles there are
examples where the interaction is very weak or nonexistent. You might have heard of neutrinos, this is
a ghost-like particle that permeates the universe and hardly interacts with familiar matter
at all, yet it is part of our family of particles that we know exist and that we can detect
and interact with. So if we can have an elusive particle that's
part of our own familiar family of particles it's not much of a stretch to think of a whole
other category of particles where none of them give a rat’s ass about the rest of
us, and they just pass right through us as though we're not even there. Now, here's what's interesting about dark
matter: we know it doesn't interact with us except gravitationally. By the way, what do I mean by interact? Does it bind and make atoms and molecules
and solid objects? No, it does not interact with us in any important
known way. But it also doesn't interact with itself. That's what's interesting. So if it interacted with itself you could
imagine finding dark matter planets, dark matter galaxies, because to interact with
yourself is what allows you to accumulate and have a concentration of matter in one
place versus another. These are the atomic bonds and the molecular
bonds that create solid objects. And if particles do not interact with one
another, they just pass through, you just have this zone of mass not really doing anything
interesting. So dark matter not only doesn't interact with
us, it doesn't interact with itself. And that's why when we find dark matter across
the universe it's very diffusely spread out. It's like over here. It's not in this one spot and look at this
concentration. No, that's not how that works. An example of this: What is a rock? It's a collection of atoms and molecules that
are stuck together by electromagnetic forces. We don't think of them that way, we just think
of them as rocks, but they are held together by forces on the atomic and molecular scale,
and they bind together and then we get what we call the solid object called a rock. If those forces did not work on you as a particle,
you have no occasion to bind with any other particle. You have no occasion—you have no ambitions
of ever becoming a solid object. It's a different kind of world, that would
be. We would not expect that world to have life
as we know it, because life requires an assembly of molecules to turn it into some separate
entity distinct from everything else that then has fascinating chemical properties that
we call life.
