Dark matter (blue) from lensing, very hot gas (red) via X-rays (Chandra)

In 1933, Fritz Zwicky, studying various star clusters, noted that the general averaged behavior of stars in the cluster generally indicated that visible stars made up only 10% of the required mass. In 1975 Vera Rubin presented results of careful measurements of the orbits of stars in nearby galaxies, and showed conclusively that 90% of the mass of a typical galaxy is not contained in stars, but in a halo of “dark matter” extending out 10 times beyond the galaxy radius. Today it is easy to use gravitational lensing to map the dark matter distributions of galaxies and galaxy clusters in great detail.

But what the hell is Dark Matter??? It has to consist of single particles, presumably fermions, with mass, but interacting with other matter only via gravity and perhaps the weak interaction. THERE ARE NO SUCH PARTICLES IN THE STANDARD MODEL. We desperately need to detect them here on earth, to learn more about them. The best way is to get very sensitive, very large detectors as deep underground as possible, so that only neutrinos and dark matter particles can get at them.

In the 1920s Einstein invented a property of empty space that gave it a tendency either to expand or contract, as if it had internal potential energy. He did this so that the universe described by his theory of gravity, which turned out to be unstable, would instead be static and eternal. But then, in 1929, Edwin Hubble discovered that the universe is in fact expanding, and Einstein said, "This is the greatest physics blunder I have ever made. If I had trusted my own equations I could have predicted an expansion!" However, in 1998, careful study of a type of supernova which has standard brightness and can be observed at huge distances, made it possible to measure the actual rate of expansion of the universe at very large distances, and it was found that space does have a property such as Einstein guessed, now called "dark energy," which causes explosive expansion of empty space... and the more empty space, the more rapid the expansion!

The discovery of dark matter and dark energy solved a puzzle. Physicists had long assumed that the universe should have a total energy of zero, much as if it were completely empty, but it never worked out that way. Now, including dark matter and dark energy as well as matter and bosons, it does work out with great precision to zero! Why should it have been zero? Because the universe would have to start with a boson of space-time popping out of nothing... not vacuum, but nothing. In order for this universe-boson to survive an indefinite time, by ΔE Δ t ∼ ℏ, it would have to have Δ E = E = 0! And it does seem to. NOW WE NEED A QUANTUM THEORY OF SPACE TIME.

A lot of amazing things happened in the very early universe, including a phase transition called "inflation" which is actually the key event in forming the universe as we see it. The only way to observe the very early universe, which was opaque to light, is to take advantage of another discovery of Einstein, gravitational radiation!