Dark Matter Yields No Secrets

A new study contradicts a key assumption about the mysterious matter that may make up much of the universe

Dark matter neither emits nor scatters light. In other words, it is completely invisible and cannot be detected through any direct means of observation available to us. However, the necessity of suggesting it exists has been forced upon astronomers by the gravitational effect it seems to have on ordinary matter in its vicinity. Such measurements suggest that dark matter is far more ubiquitous than the matter we are used to, and makes up nearly 83 per cent of all matter in the universe. But, so far, attempts to categorise the constituents of dark matter have not yielded success. The latest study of the phenomenon has only added to the mystery.

According to a Harvard-Smithsonian Center for Astrophysics release, “Now, we know less about dark matter than we did before,” lamented Matt Walker, lead author of a report on the work to appear in The Astrophysical Journal.

Models developed by cosmologists predict that dark matter consists of some sort of exotic particle that, through gravity, clumps together with others of its kind. These tiny grains are believed to be sluggish, so dark matter is typically dubbed ‘cold dark matter’, slow particles being normally associated with coldness.
Over cosmic history, cosmologists think clumps of dark matter grew and attracted normal matter, forming the galaxies. Cosmologists use powerful computers to simulate this process. Their simulations show that dark matter should be densely packed at the centres of galaxies. “If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter ‘pit’ at the centre,” explained Jorge Peñarrubia of University of Cambridge in the UK, co-author of the new study.

But measurements of two dwarf galaxies show they contain a smooth distribution of dark matter, Walker said. So, “our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies.” In other words, “the first two dwarf galaxies we studied are like pitless peaches,” said Peñarrubia.

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