Dark matter remains one of the biggest unsolved mysteries in modern science. For many years it has been well established that visible matter accounts for only a tiny fraction of the total mass of the Universe. This has been most recently confirmed by measurements of the cosmic microwave background taken by the WMAP satellite, which has given us accurate figures for the density of matter in the Universe, supporting earlier evidence from studies of big bang nucleosynthesis and large scale structure, that a substantial fraction of the Universe is made up of non-baryonic dark matter. Despite this progress we still don't know exactly what this is; and we can't be sure that it exists at all.
Astronomical measurements show dark matter also dominates on smaller scales. Measurements of the rotation curves of spiral galaxies, and the velocities of galaxies within clusters suggest that they are embedded in dark halos of invisible matter, containing most of the mass of the galaxy.
Particle physics has provided a possible explanation for non baryonic dark matter in the form of weakly interacting massive particles (WIMPs). The most likely WIMP candidate is the neutralino, predicted by supersymmetry theory. Supersymmetry is popular theory as it can explain the hierarchy problem - the difference in scale between electroweak interactions and the Planck mass. It also predicts the existence of a stable particle, the neutralino, with all the properties of an ideal dark matter candidate.
Although neutralinos have remained the most likely explanation for dark matter for some time; this is still just a hypothesis. There are many other explanations for dark matter, including other WIMPs (such as Kaluza Klein particles) and alternative theories of gravity (such as Modified Newtonian Dynamics (MOND)). Therefore we need to test this hypothesis with a direct detection experiment.
EURECA is designed to do this by searching for the elastic scattering of WIMPs of atomic nuclei. The scattering cross section is expected to be very small - down to 10-10 pb, making this a challenging but not impossible task.