The canonical cosmological constant dominated cold dark matter model ( \Lambda CDM ) may possess too much power on small scales at z = 0 , manifested as central over-concentration of dark matter and over-abundance of dwarf galaxies .
We suggest an alternative model , \Lambda DCDM , where one half of the cold dark matter particles decay into relativistic particles by z = 0 .
The model successfully lowers the concentration of dark matter in dwarf galaxies as well as in large galaxies like our own at low redshift , while simultaneously retaining the virtues of the \Lambda CDM model .
The model solves the problem of over-production of small dwarf galaxies in the \Lambda CDM not by removing them but by identifying them with failed , “ dark ” galaxies , where star-formation is quenched due to dark matter evaporation and consequent halo expansion .
A dramatic difference between the \Lambda DCDM model and other proposed variants of the \Lambda CDM model is that the small-scale power at high redshift ( z > 2 ) in the \Lambda DCDM model is enhanced compared to the \Lambda CDM model .

A COBE-and-cluster normalized \Lambda DCDM model can be constructed with the following parameters : H _ { 0 } = 60 km/sec/Mpc , \lambda _ { 0 } = 0.60 , \Omega _ { 0 ,CDM } = 0.234 , \Omega _ { 0 ,b } = 0.044 , n = 1.0 , and \sigma _ { 8 } = 1.06 .
A clean test of this model can be made by measuring the evolution of gas fraction in clusters .
The prediction is that the gas fraction should decrease with redshift and is smaller by 31 \% at z = 1 than at z = 0 .
X-ray and Sunyaev-Zel ’ dovich effect observations should provide such a test .