Composition Until 30 years ago, we thought all matter was “baryonic” matter (protons, neutrons, electrons). Now: 4.6% is baryonic matter 95% is non-baryonic matter

Non-Baryonic Matter 23% is Dark Matter- composed of sub-atomic particles that interact weakly with ordinary matter. 72% is Dark Energy - this amount would explain a flat, accelerating universe… which is what we observe. ??% are neutrinos - like electrons, but without a charge.

Dark Matter Measure mass of galaxies and stars by how fast objects orbit them. Galaxies have more mass than stars, gas and dust within them. Also, gravitational lensing shows too much mass in galaxies.

Dark Matter Gravitational pull, but does not emit or absorb light. Could be… 1.Brown Dwarfs: small stars too small to shine, but still have large gravity. - called MACHOS (massive compact halo objects). If so, than missing mass is baryonic.

Dark Matter 2.Supermassive Black Holes: May power quasars. May be detected through lensing. 3.New Forms of Matter: Called WIMPS (Weakly Interacting Massive Particles)… non- baryonic matter.

Dark Energy New cosmological constant… first used by Einstein to keep universe static… now used to explain acceleration. Cosmological Constant = energy from vacuum of space. Quantum fluctuations. Something from nothing? Consistent with observations