Science & Environment

How does new dark matter evidence fit together?

Dark matter map - DES Image copyright Dark Energy Survey
Image caption One team has carried out large-scale mapping of dark matter in the sky

In recent weeks there have been an unprecedented number of papers on dark matter in such a short time.

They comprise new gravitational lensing experiments that have mapped dark matter on a range of scales.

These all use similar methods, but vary in terms of resolution and size. Similar to the art of making maps on Earth this "skotocartography" tells us different but complementary things about the nature of dark matter.

The skoto- prefix comes from the Ancient Greek word "skotos", meaning "darkness".

The dark energy survey study by Vinu Vikram et al. has made the largest of these maps. In this we see the general "lay of land", equivalent in a cartographic sense to finding where there is land, and where there is sea.

They find that galaxies tend to exist in environments where the general dark matter density is high.

The Hubble Space Telescope and Chandra study by David Harvey et al. focused in on galaxy clusters and looked at the mergers between these clusters. This is like making a road-map of the UK, marking out where the cities and large towns exist.

Image copyright ESO/R. Massey
Image caption The most recent study mapped the dark matter in four colliding galaxies

We can learn more about the dark matter properties in this way, but lose the large-scale correlations. We found that dark matter flits through itself, hardly interacting at all.

In the new European Southern Observatory study by Richard Massey et al. we found a single particularly interesting galaxy cluster, zoomed in, and made a map of very fine detail.

This is like creating an A-Z map of a single city or town. In doing this we learnt even more about how dark matter is behaving in that cluster; but traded this off against a larger statistical sample.

We found that dark matter might not flit through itself completely, but might be a very tiny bit sticky.

This art of dark matter map-making is in a golden age. As the data continues to accumulate we will map more and more of the cosmos.

Eventually as we complete our maps we hope that the mystery of dark matter will be solved.

Dr Thomas Kitching tweets at @tom_kitching.