Good question! We have lots of reasons to believe that matter which is
made up of atoms makes up only a tiny fraction of the total matter and
energy in the universe -- about 4% according to current estimates.
There are lots of measurements that confirm this.
One of the older ones which is quite compelling is just looking at
how fast stuff is going as it orbits the centers of galaxies. Most of
the stars in a galaxy are near the core, and the density of stars falls
off as you get farther away from the core of a typical galaxy. Even if
we don't understand what makes up the matter of the core (stars, black
holes, what have you), you can still predict how the rotation speed
will vary with the distance from the core, because we can estimate how
much stuff there is outside the core, in the arms. Then we can measure
the rotation speed everywhere in a galaxy by looking at the Doppler
shifting of the light coming back. What we get doesn't match with the
assumption that the matter is in the core and in visible matter in the
arms/disks of galaxies. It's a big deal, too -- the estimations are off
by a factor of four or more for most galaxies. Dark, unseen matter
clumping around galaxies would help solve the problem.
Also, measurements of the large scale structure of galaxies and
clusters of galaxies indicates that additional matter must be present
to provide the gravitational attraction to clump the matter on the time
scales from the beginning of the universe until now. Measurements of
the cosmic microwave background and supernovas give very precise
estimates of the age of the universe and the cosmic microwave
background also tells us how clumpy the matter was shortly after the
big bang. It all makes sense if there is about four to five times as
much dark matter as matter which we can see.
It gets worse. The expansion of the universe appears to be
speeding up (again, the cosmic microwave background and supernova
experiments are important)! To explain this, we need to introduce
another component of the matter and energy balance of the universe.
About 73% of everything is required to be a negative-pressure, "dark
energy" stuff. 23% of everything is dark matter, and 4% is the familiar
atoms we know and love.
We don't know what stuff makes up the dark matter and we wish very
much that we did. It's not neutrinos, because these zip away from
galaxies and don't orbit them. Whatever the dark matter is, it doesn't
interact with ordinary matter very much or at all (except through
gravitational attraction). We hope that it interacts via the weak
nuclear force so we have a chance of detecting it in laboratory
experiments. Some hypotheses of what dark matter is are supersymmetric
partners of ordinary, known particles (the partner of the photon and
weak force carriers is an attractive candidate for a dark matter
constituent), or axions (another neutral, weakly-interacting
hypothetical particle), or perhaps anything else exotic you can think
of. I have no idea what the dark energy consists of.
(published on 10/22/2007)