So, what could it be? If the baryons 
account for about 4.5% of all matter 
density, and the total matter density 
corresponds, corresponds to omega of, of 
27% of the critical density, what are 
they? They're not baryons, they has to be 
something else. 
There are many different possibilities, 
one of which is massive neutrinos and 
these are the only particles that 
contribute to the dark-matter that we 
actually know exist, but they do not add 
up to very much. 
They probably add up to less mass than 
all mass in stars. 
There have been many, many suggestions 
about possible constituents of the 
dark-matter and, among them, two 
scenarios emerged as the most likely 
ones. 
One is a generic kind of particles called 
WIMPs for Weakly Interacting Massive 
Particles, the acronym was obviously 
played too much as for massive compact 
halo objects. 
We do not know what WIMPs would be, but 
we have some ideal what their masses 
could be. 
There are theories that go beyond the 
Standard Models of partical physics like 
Supersymmetry which predict the existence 
of new kinds of particles, some of which 
could indeed be the dominate constituents 
of the dark-matter. 
Right now, it is not known which if any 
of those are true, 
but we're trying to find out. 
From the accelerator measurements, we 
have fairly good limits on what their 
masses can be, and it seems to be that, 
that they're at least tens of times the 
mass of a proton. 
There's a hope that direct detection of 
particles responsible for the dark-matter 
will be made in accelerators like large 
[UNKNOWN] collider. 
But there are also laboratory experiments 
trying to detect them directly, I'll talk 
about them in a second. 
There are more speculative possibilities 
of particles not known to exist, but that 
can be concocted some way or other. 
WIMPzillas is one of those, those would 
be really, really massive particles, 
early thermal remains from the Big Bang. 
Another generic kind of possible 
dark-matter particles are called axions 
and they occur in some of the theories. 
They could be much lighter, 
but they can also be massive and there 
are also experiments that can be designed 
to place limits on their density. 
But there are many, many other 
possibilities that have been put forward, 
probably hundreds. 
in fact, here is a table from an old 
review paper and I'm reproducing it here 
just to show you the broad variety of the 
kinds of possible constituents to 
dark-matter. 
One thing that you can notice if you look 
through this table, that the mass range 
of possible dark-matter constituents is 
80 orders of magnitude, which was simply 
indicative of our complete ignorance of 
what they can be. 
Many of these have been eliminated since 
then through variety of measurements and 
WIMPs remain probably the most likely 
kind. 
Well, what kind of types of dark-matter 
there can be, 
there can be and probably is more than 
one type of particles involved. 
For sure there are neutrinos and then 
there is something else, but that 
something else maybe more than one type 
of particles. 
Remember, 
there used to be only a few chemical 
elements and now there are 92, and so, 
there may be 92 different kinds of 
dark-matter, we don't know. 
But for simplicity, we'll start by 
assuming there is only one. 
Now, what can we say about properties of 
these particles? Turns out that their 
mass influences the way the large scale 
structure in the universe forms, and 
therefore, observations of the large 
scale structure can constrain physics of 
the dark-matter particles. 
The basic generic difference is are there 
small mass particles, which move at 
relativistic speeds or as we call hot 
dark-matter or are there more massive 
particles that move at slower speeds, 
which are called cold dark-matter. 
As it turns out, 
the hot dark-matter leads to a structure 
formation where the biggest structures 
form first, because the, all the small 
scale irregular, irregularities are 
erased and we'll talk about that later in 
class. 
Whereas, cold dark-matter makes 
structures form at all scales more or 
less simultaneously, then smaller ones 
merge to make larger ones. 
That seems to be a much better agreement 
with observations. 
So nowadays, aside from small 
contribution of massive neutrinos, the 
generic belief is that most of the 
non-baryonic dark-matter is of the cold 
variety, more massive particles. 
I mentioned the attempts to detect 
dark-matter directly and there are 
several of those going on. 
The idea here is if you have enough 
practice, 
you are usually deep underground in one 
of the underground laboratories in order 
to eliminate foreground effects of cosmic 
rays, so properly shielded and only 
dark-matter can go through, because 
evidently, it doesn't interact with 
regular matter in any other way than 
gravitationally. 
Some of the dark-matter particles might 
interact with the material in your 
detector. 
These are essentially really, really 
precision, high precision [INAUDIBLE] And 
the idea here is the simply mechanical 
encounter of a dark-matter particle, with 
say an atom which, are super cold 
germanium conveys enough kinetic energy 
that can be in principal detected by this 
apparatus. 
So they integrate for a long time, they 
have to worry a lot about eliminating 
possible foreground signals. 
And the only way that you can tell that 
you are actually looking at dark-matter 
particles is to look for the annual 
modulation. 
It's probably good assumption, that the 
halo of the milky way doesn't rotate very 
much and sun goes around the galactic 
center. 
So there is a wind of dark-matter coming 
through the solar system in the opposite 
direction of our solar motion. 
Now, the earth goes around the sun, and 
so the velocity vector sometimes adds and 
sometimes subtracts with a 12 month 
modulation. 
So, if you can see a signal that is 
modulated in sjuch a way, that we see 
more encounters when earth's velocity 
vector is alligned with solar velocity 
vector relative to the galactic center 
then, and then you see the exact opposite 
6 month later. 
That will be a pretty good indication 
that you're actually detecting 
dark-matter particles. 
Claims have been made in the literature 
by one group to have actually seen such 
an effect, but none of the other groups 
can reproduce it, 
and at this time, it is just generally 
not believed that deduction has been made 
yet. 
But it could happen any day, 
this is a first rate experimental physics 
experiment and there are some very 
capable people working on it. 
So, we may discover dark-matter particles 
in underground laboratories really soon. 
Well, the other option is that there is 
no dark-matter, 
that we misinterpreted gravity, that the 
law of gravity is different at large 
distances or rather at small 
accelerations. 
The dominant of those is so called MOND 
for modified Newtonian dynamics, 
theory due to Milgrom. 
It is very much ad hoc, 
it was invented only to explain the 
rotation curves of these galaxies. 
As it turns out, it actually runs into 
problems with observations with clusters 
of galaxies or even actually globular 
star clusters and its proponents keep 
tweaking it up. 
So it does seem to be always adjusted to 
fit observations for these galaxies and 
they're efficient, 
but it's always running in some different 
problem. 
And most astronomers do not believe that 
this is a viable possibility. 
You may think that life is a sligh, any 
less believable than inventing some new 
kind of particles, 
but we do know that a lot of different 
kinds of particles, most of which do not 
interactive electromagnetically. 
And so, why not? It's preferable to be 
sensible. 
We see gravitational effects that can be 
easily understood with the presence of 
the dark-matter and there are many good 
arguments for its existence, not just the 
measurements that I'd mentioned earlier 
but there are also implications for 
structure formation that we'll cover a 
little later in the class. 
so, vast majority of cosmologists and 
astronomers believe that, yes, there is 
dark-matter of some kind. 
We're just trying to find out what 
exactly is it made of and that MOND is 
not really surviving the observational 
tests very well. 
One interesting observation is that the 
relative fraction are importance of 
dark-matter seems to increase with the 
radius. 
You may recall that in near the centers 
of galaxies, all of the rotation curve or 
motions of stars or gas can be explained 
by the enclosed mass just in stars are. 
But thus you go to larger and radii, 
larger radii, stellar density decreases, 
the, the relative importance of the 
dark-matter rises. 
And that's certainly true in scaled 
galaxies up to some tens of kiloparsecs 
in radius, 
turns out it extends all the way up to 
about megaparsecs in radius, and then it 
remains constant. 
So, once we reach the scales of clusters 
of galaxies or beyond, the mix of visible 
and non-baryonic dark-matter seems to be 
constant. 
Next time, we'll talk about gravitational 
lensing, which is a completely 
independent way to constrain the amount 
of a gravitating mass in the universe.