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