In the early 1970s, the American astronomer Vera Rubin studied the rotation of several nearby spiral galaxies, including the Andromeda galaxy. Vera noticed that stars in the outer arms of these galaxies orbit their galactic centres faster than calculations would predict, based on estimates of the combined gravitational mass of all of the observable material within each galaxy.
In order to explain this discrepancy, theorists suggested that galaxies might actually contain much more material, by mass, than is directly observable to us as stars, gas and dust. This theoretical material became known as ‘dark matter’ – material that is invisible to us because it doesn’t interact with light, or any other part of the electromagnetic spectrum. It is currently believed that up to 90 per cent of a galaxy’s mass is in the form of a spherical halo of this dark matter, extending out far beyond the boundary of the galaxy’s visible disk of ‘luminous’ matter.
Astronomers do not currently know what this theoretical dark matter consists of. However, many candidates exist, which can generally be categorised as ‘cold’, ‘warm’, or ‘hot’ dark matter – see below.
Alternative theories, to explain the phenomenon, include proposed modifications to the known laws of gravitation. These theories suggest that gravity’s simple ‘inverse square law’ would need an additional term, which would make the gravitational interaction stronger than otherwise expected, over galactic distances. It should be noted that the need for a modified theory of gravitation has not yet been ruled out.
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