Astronomy 162:
Introduction to Stellar, Galactic, & Extragalactic Astronomy

Lecture 43: Dark Matter

Key Ideas:

Dark Matter is matter we cannot see directly

Evidence for Dark Matter:

Baryonic & non-Baryonic Forms

Galaxy Rotation Curves revisited

Spiral Galaxies rotate such that the rotation speed:


Mass Distribution in Galaxies

Most of a galaxy's light is in the inner 10 kpc

If stars provided all of its mass we expect:

But the curve stays flat!

Dark Matter Halos

Q: Where is the extra mass if it is not coming from stars?

A: Galaxies have extended "dark halos"


Fritz Zwicky's "Missing Mass"

1933: Fritz Zwicky measured the peculiar velocities of galaxies in the Coma cluster

Zwicky suggested that "missing mass" adds extra gravity to hold the cluster together.

Dark Matter in Clusters

Subsequent observations showed that galaxy clusters are 90-99% dark matter.

More evidence:

Dark Matter

It is "Dark" because it cannot be detected directly using light (of any kind).

Detected only because of the effects of its gravity on its environment:

What is it?

Baryonic Dark Matter?

Non-Baryonic (Particle) Dark Matter?

Some mixture of the two?

Baryonic Dark Matter

Some ideas:


Microlensing Searches

What if a Brown Dwarf, Jupiter, or dark stellar remnant in the halo passes between Earth and a more distant star?

Gravitational Microlensing

Microlensing Collaborations

Chance alignments are very rare.

Need to monitor millions of stars for many years.

Non-Baryonic Dark Matter

Fundamental particles which only interact via gravitation or the weak force.

Massive neutrinos:

Exotic new particles:

Particle Dark Matter Searches

Searches for direct detection of dark matter particles.

These include:

A (Very) Radical Suggestion:

Some have suggested that Dark Matter does not exist, but rather our theory of gravity is wrong on large scales!


How much Dark Matter is there?

D and He from Primordial Nucleosynthesis predicts W in baryons alone is


Galaxy halo masses from rotation curves gives



Updated: 1997 November 24