Dark matter is everywhere

An unfortunate thing about science is that important discoveries take a long time to hatch.

They can be utterly ignored for decades, until a new discovery makes the earlier work impossible to ignore. Even scientists get selective blindness and deafness regarding work that contradicts their favourite theories.

The research done by Fritz Zwicky in 1933 and Vera Rubin in the 1960s are really good examples of this. They were ignored for years but eventually became fundamental to our understanding of the universe.

On any summer or autumn night, we can look at the Milky Way and see dark rifts in it. These are not rifts, they are great clouds of gas and dust, the raw materials for making new stars and planets.

However, in a galaxy, how much of this invisible stuff is there? How can we find out?

In 1933, Zwicky had an idea. He looked at a cluster of galaxies and by measuring the amount of starlight, estimated the number of stars and thence the amount of material in each galaxy.

He then observed the orbital velocities of the cluster members as they were gravitationally tugged by all the others.

Since gravity is produced by mass, this would give him the mass of the whole cluster. By subtracting the total mass tied up in stars in all the cluster members from the total mass of the cluster, he could estimate the amount of gas and dust.

However, the answer did not make sense. It suggested there was far too much material to account for by the gas and dust. The research community did not believe this result, and the issue was dropped for more than 30 years.

In the 1960s, Rubin chose a nice non-controversial project to work on. She measured the motions of stars in our galaxy, the Milky Way, as they moved in their orbits, in order to determine the gravitational force acting on them and from that estimate the mass of our galaxy.

She then totalled up the mass of matter in stars and as Zwicky had found, got an answer that did not make sense. The mass was so high that what was visible accounted for only 5% of it.

As in the case of Zwicky's discovery, the research community ignored her work. So she decided to take her research further. She looked at another galaxy, the Andromeda Galaxy, which closely resembles our galaxy and close enough to study in detail.

She measured the brightness of the starlight at different distances from the core, which gave her an estimate of numbers of stars and how much material they contain.

She then used these mass estimates to calculate how fast the material should be orbiting the core of the galaxy and compared it with the observations. They did not agree; the material was orbiting far faster than it should.

This meant the gravitational attraction was stronger than it should be, and since gravity is a force due to mass, there was much more material in our galaxy than expected.

The discrepancy was huge, suggesting an amount of additional mass impossible to account for in terms of gas and dust clouds, or black holes. Once again, nobody took much notice of these important results.

The situation was changed completely by researchers working in a different area, the origin of structure in the universe.

They found that to account for the structures we see in the early universe, and the large-scale distribution of galaxy clusters today, the matter we can see can only be about 5% of what is really there. This invisible material has become known as dark matter.

This completely vindicated the work of Zwicky and Rubin, and underlines the value of looking deeply at the scientific questions nobody else is looking at, even if they are currently uncontroversial. So far, we have no idea what dark matter is.

• Brilliant Jupiter lies in the south after dark
• Saturn is low in the southeast.
• The moon will be new on the 31st.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.