Mass-to-Light Discoveries

When astronomers measured the galactic rotation curve for the Milky Way, the rotational velocity did not decrease exponentially with distance -- it actually increased, then settled to a near-constant value. So they concluded that most of the galactic mass was located at the edges of the galaxy (outside the sun's orbit of 28,000 light years from the center), or in the halo portion. The outer regions and the halo portions of the galaxy emit very little light. Therefore, whatever mass is in these regions (and there's lots of it) is dark, hence the term "dark matter." In fact, there is six times more dark matter than light matter in the Milky Way.

pie chart
NASA/WMAP Science Team
The composition of the universe

This discovery of a high mass-to-light ratio for the Milky Way was not altogether new. In 1933, astronomer Fritz Zwicky used similar methods to measure luminous mass and rotational mass in clusters of galaxies, (large swarms of galaxies that orbit each other). He found mass-to-light ratios that were greater than 100. Zwicky suggested that the differences between total mass and luminous mass were due to dark matter. His findings were not well received by most astronomers, but today the idea that dark matter exists is generally accepted.

In the 1960s, astronomer Vera Rubin made a rotational curve for the Andromeda galaxy (M31) and found a similar pattern to that observed in the Milky Way. She and her colleague, Kent Ford, made rotational curves for several spiral galaxies and found curves similar to that of the Milky Way. The implication of all of these results pointed to two possibilities:

1. Something was fundamentally wrong with our understanding of gravity and rotation. This was not likely because Newton’s laws have withstood many tests for centuries -- they apply to most situations except objects traveling near the speed of light or in extreme gravity, in which case Einstein’s theories of special and general relativity apply (see How Special Relativity Works).

2. Galaxies and galactic clusters must contain far more dark matter than light matter.

Astronomers can detect dark matter by examining the X-rays it emits. In the next section, we’ll find out how dark matter can also bend light.