Known Contents of the Universe

• Large objects: Various types of stars (luminous matter); Star remnants including black holes, both star-sized and supermassive.
• Smaller objects: Planets; Meteoroid- or asteroid-sized objects; Gas and dust, loose in the interstellar or intergalactic medium, or in denser clouds.
• Particles and radiation: Electromagnetic and gravitational waves; cosmic rays; neutrinos; ...
• Are we missing anything? Yes, about 95% of what the universe is made of! There is matter whose only detectable effect is gravitational and cannot otherwise be seen (yet?); This has happened before (think of how Neptune or Sirius B were discovered), so this "dark matter" may or may not be of a new type.

 Evidence for Dark Matter

• Inside a galaxy: The rotation curve problem for stars (spiral and elliptical galaxies) and H clouds (spiral galaxies, like NGC 5746) [and the bobbing motion problem in the Milky Way].
• In clusters: Evidence for more matter from motion of binaries and around the center; X-ray emission from hot (many millions of K) intergalactic medium (e.g., in the Hydra cluster); and gravitational lensing (images are distorted by the lens), which even allows us to map the dark matter.
• Conclusion: The mass-to-light ratio indicates that 90% of the matter may be invisible (dim, and/or of a new type), at least for spiral galaxies; We are seeing only the tip of the iceberg.
• Is there an alternative way out? Maybe the laws of gravity need to be modified; This is a possibility one should consider carefully; Most scientists think it is unlikely, and it is not clear whether it fits observations.

What Is the Dark Matter?

• Ordinary matter: Is it known matter? Subatomic particles (like baryons or neutrinos), gas clouds, MACHOS (like dwarf stars or planet-sized objects)? Whole galaxies?
• Non-ordinary matter: Is it new, unkown kinds of matter (neutralinos, WIMPS, Q-balls made of supersymmetric particles)? Hot or cold?
• Search: Use microlensing to look for MACHOS; only part of the missing mass can be accounted for; Search for new types of particles on Earth, using detectors deep underground, and for information from space that is not provided by electromagnetic waves.

Additional Twist

• Dark energy: The most distant supernovas show that the expansion of the universe is accelerating; Dark matter is not the whole story, and there is an even bigger repulsive "dark energy", possibly Einstein's "cosmological constant";

  Current Opinion

• Composition: There is probably more than one kind of dark matter ("hybrid model", with possibly 80% cold dark matter in galaxies, and additional hot dark matter at the galaxy cluster level); Total about 23% of the content of the universe.
• Does it make a difference? The type of matter affects our views on when stars and galaxies form, and the way clusters, superclusters and voids evolve; The total amount of matter also affects our predictions for the future of the universe.
• Predictions: Recent measurements by satellites confirm ideas about the composition of the universe, and that it will expand forever (regular + dark matter together add up to only 27% of what is needed for it to eventually recollapse), possibly accelerating (dark energy is repulsive, and is 73% of the content of the universe!)
• Plans: The proposed SNAP (Supernova/Acceleration Probe) mission will study the recent expansion history of the universe, and should be able to distinguish even better between different possibilities.