From: "John B."
Date: 3 Nov 2009

Q If dark matter has not been directly observed, why do scientists feel the need to propose its existence?

A Thanks for asking! This is probably the most frequently-asked of all questions about dark matter, and understandably so. On the face of it, the dark matter hypothesis is a hard one to swallow; it proposes that the vast majority of the matter in our universe is composed of mysterious stuff that does not resemble any terrestrial matter and has never been directly observed. Nonetheless, there is wide agreement among cosmologists today that some form of dark matter really exists. How have so many people come to such an extraordinary conclusion?

To help answer this central question, we have posted a series of essays on our main Education page entitled

Motions of matter

We can learn about the universe today by watching the motions of its visible parts: gas, stars, galaxies, and galaxy clusters. Using the laws of motion and gravitation determined by Newton, Einstein, and others, we can estimate an object's mass by measuring the effect of its gravity on other objects nearby. We find that the orbital speeds of stars and gas clouds in galaxies are so fast that the gravity of the visible matter is not enough to hold them together. There must be additional, invisible mass to provide the extra gravitational pull needed to hold the structure together. Similarly, the motions of gas and galaxies within galaxy clusters tell us that these larger structures must also be dominated by "dark matter".

Gravitational lensing

When light from a distant source passes near a heavy object, its path is bent slightly by gravity. This phenomenon of "gravitational lensing" has been used to map out the mass distribution around galaxy clusters, based upon the distortions induced in the images of more distant galaxies. Again, we find that the visible objects in these clusters are only a small portion of the total mass.

The universe's baby pictures

We can also learn about the universe by studying evidence from its earliest history. Two sorts of data about the early universe are particularly important:

1. Primordial nucleosynthesis: During its first few seconds of existence our universe was as hot as the interior of a star, hot enough for nuclear fusion to take place. Certain light nuclei (notably deuterium and helium) were produced in these first moments, and by studying their abundances we can learn about the conditions in that primordial fireball. These data give clear evidence that the bulk of the universe's matter is "non-baryonic" in nature - that is, it is made of something different from the protons, neutrons, and electrons that build up all matter on earth.
2. The cosmic microwave background: Some of the heat from the early universe is still detectable in the sky today, visible as microwave radiation coming nearly uniformly from all parts of the sky. The study of this snapshot of the early universe has already lead to two Nobel Prizes in Physics. The pattern of slight hot and cold spots in the microwave background across the sky also tell us about the properties of the early universe, and lead us to the same conclusion: much of the universe's matter is non-baryonic in nature.

Evolution of the universe

Finally, the composition of the universe determine how the massive structures within it - galaxies and galaxy clusters - grow and change over time. The uniformity of the microwave background radiation tells us that the distribution of baryonic matter in the early universe was extremely uniform. Without the presence of dark matter to provide additional gravitational pull, there simply has not been enough time for the smooth early universe to grow into the very lumpy one we see around us today.

Thanks again for the question, and please see the rest of this site for further information.

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Dr. Jeffrey Filippini
Observational Cosmology Group
California Institute of Technology
(formerly of University of California - Berkeley)
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