Most of the matter in the Universe is dark and completely different in nature from the matter which makes up stars, planets and people. Galaxies formed and grew when gas cooled and condensed at the centre of enormous clumps of this dark matter, the so-called dark matter haloes. Astronomers can infer the structure of big dark matter haloes from the properties of the galaxies and gas within them, but they have no information about haloes that might be too small to contain a galaxy.

In results just published in Nature, an international research team, led by Prof. Jie Wang from National Astronomical Observatories of Chinese Academy of Sciences (NAOC), harnessed supercomputers in China and Europe to zoom into a typical region of a virtual universe by an amount equivalent to zooming into an image of the Moon so as to recognize a flea on its surface. This allowed them to make detailed pictures of hundreds of virtual dark matter haloes from the very largest to the very smallest.

The biggest dark matter haloes in today's universe contain huge galaxy clusters, collections of hundreds of bright galaxies. Their properties are well studied, and they weigh over a quadrillion (a thousand billion) times as much as our Sun. On the other hand, the masses of the smallest dark matter haloes are unknown. They are hypothesised to be about the mass of the Earth by the currently popular theories for the nature of the dark matter that underlie the new supercomputer zoom. Such small haloes would be extremely numerous, containing a substantial fraction of all the dark matter in the universe, but they would remain dark throughout cosmic history because stars and galaxies grow only in haloes more than a million times as massive as the Sun.

The research team, based in China, Germany, the UK and the USA took five years to develop, test and carry out their cosmic zoom which covers a range of 10 to the power 30 (that is a 1 followed by 30 zeroes) in mass. This is equivalent to the number of kilograms in the Sun. It enabled them to study the structure of dark matter haloes of all masses between that of the Earth and that of a big galaxy cluster. Surprisingly, they found them all to have very similar internal structure, very dense at the centre, becoming increasingly diffuse outwards, and with smaller clumps orbiting in their outer regions. Without a scale-bar it is almost impossible to tell a dark matter image of the halo of a massive galaxy from one of a halo with mass a fraction that of the Sun.

The result has a potential practical application. Particles of dark matter can collide near the centres of haloes, and may, according to some theories, annihilate in a burst of energetic (gamma) radiation. The new zoom simulation allows the expected amount of radiation to be calculated for haloes of differing mass. Much could come from haloes too small to contain stars. Future gamma-ray observatories might be able to detect this emission, making the small objects individually or collectively "visible". This would confirm the hypothesised nature of the dark matter, which may not be entirely dark
after all!

The simulations were carried out in the Cosmology Machine supercomputer at Beijing in China, Durham in UK, and Munich in Germany.

The paper can be accessed at ………….