It is the ratio of the density and the critical density. The cosmological standard model gives an Omega close to 1, indeed equal to 1. The existence of non baryonic dark matter is deduced from these assumptions. The second one called Cold Dark Matter is made of weakly interacting massive particles , the so called WIMPs . At the galaxy scale, Dark Matter is supposed to form a spherical halo.
WIMPs interact by elastic scattering on a nucleus in a crystal.
So, very special detectors are needed in an ultra-low background environment. Long observations are done on a large number of stars in the Large Magellan Cloud. No experiment in direct or indirect detection has showed a WIMP signal yet.