Visible matter - stars, planets and interstellar gas - accounts for only about five percent of the matter content in the Universe. Everything that remains is invisible: about three quarters Dark Energy and just under one quarter of Dark Matter, identified only through its gravitational pull. A direct detection of Dark Matter is still outstanding, its properties, however, are crucial to the formation of structure in the Universe. They predict how haloes of Dark Matter form - Galaxies evolve inside these Dark Matter haloes - and how the haloes are distributed in the Universe. The process of cosmic structure formation has been studied at the MPA with the Millennium simulations in unprecedented accuracy.

A comparison of these kinds of simulations with observations of large numbers of galaxies gives the scientists clues about how the formation of galaxies and haloes are linked. Assuming that larger galaxies form in larger haloes, one can deduce a statistical relation between visible and invisible matter in galaxies with different mass. To understand the evolution of individual galaxies in detail, simulations with much higher resolution are needed, where visible and dark matter can interact directly. In addition to gravitation many other physical processes have to be taken into account, such as the gas hydrodynamics, the thermal evolution of the interstellar and intergalactic medium, star formation and evolution and the effects of cosmic UV-radiation.

In their now published work, Till Sawala an his co-authors selected six Dark Matter haloes with different formation histories from the Millenium-II simulation and carried out new simulations with 100 times higher resolution. All six haloes grow to a mass of some 100 billion solar masses, which corresponds to galaxies of about one million solar masses in stars according to the statistical relation. The detailed simulations however yielded star masses between 50 and 100 million solar masses ? orders of magnitude more than the expected amount.

These results agree indeed with previous similar simulations of individual dwarf galaxies. The representative selection of the haloes enabled the team around Till Sawala to show now that the much larger mass in stars is not due to peculiarities of the galaxies simulated. It does in fact represent a disagreement between the current simulations and observations.

"Three explanations could resolve this discrepancy", Till Sawala comments. "The observational picture could be incomplete, which would mean that there are many more dwarf galaxies than we believe at the moment. Alternatively, the abundance of Dark Matter haloes could be different from predictions of the so-called standard model of Cold Dark Matter. If both the dwarf galaxy count and the Cold Dark Matter model are correct, then we would have to conclude that all simulations over-predict the real star formation rates in dwarf galaxies by at least a factor of 10."

All three potential solutions would entail far-reaching consequences: "We think that the current observational counts of galaxies are sufficiently complete and that we have a fairly good understanding of the remaining incompleteness," explains co-author Qi Guo of the University of Durham. "A difference by a factor of four would also affect other models, which rely on these results."

To test the second hypothesis, i.e. an alternative to the established Cold Dark Matter model, the authors compare the distribution of haloes with simulations of Warm Dark Matter. Since in this model less structure forms on small scales, it appears to bring the abundance of simulated galaxies in agreement with observations. Professor Simon White, however, is sceptical: "Such a model is in conflict with other observations which makes this explanation very unlikely."

If both the observed count of dwarf galaxies is complete, and the distribution of Dark Matter follows the predictions, the remaining option is for the simulations to be wrong about the efficiency with which stars are formed. Since all current simulations reach similar results with slightly different methods, this does not seem to be due to some numerical error. A more likely explanation is that some important process, responsible for inhibiting star formation in real galaxies, has so far been missing from the simulations.

"Only one thing is certain: The current observations of dwarf galaxies, the assumptions of how they populate dark matter haloes, and the current simulations of galaxy formation cannot all be correct at the same time", summarizes Till Sawala. The authors agree: "The study of galaxy evolution will continue to surprise us for some time to come."

Till Sawala