The canonical cosmological constant-dominated cold dark matter model (Lambd
a CDM) may possess too much power on small scales at z=0, manifested as cen
tral overconcentration of dark matter and overabundance of dwarf galaxies.
We suggest an alternative model, Lambda DCDM, where one-half of the cold da
rk matter particles decay into relativistic particles by z=0. The model suc
cessfully lowers the concentration of dark matter in dwarf galaxies as well
as in large galaxies like our own at low redshift while simultaneously ret
aining the virtues of the Lambda CDM model. The model solves the problem of
overproduction of small dwarf galaxies in the Lambda CDM, not by removing
them but by identifying them with failed, "dark" galaxies, where star forma
tion is quenched as a result of dark matter evaporation and consequent halo
expansion. A dramatic difference between the Lambda DCDM model and other p
roposed variants of the LCDM model is that the small-scale power at high re
dshift (z>2) in the Lambda DCDM model is enhanced compared to the LCDM mode
l. A COBE- and cluster-normalized Lambda DCDM model can be constructed with
the following parameters: H-o = 60 km s(-1) Mpc(-1), lambda (o) = 0.60, Om
ega (0,CDM) = 0.234, Omega (0,b) = 0.044 n = 1.0, and sigma (8) = 1.06. A c
lean test of this model can be made by measuring the evolution of the gas f
raction in clusters. The prediction is that the gas fraction should decreas
e with redshift and is smaller by 31% at z = 1 than at z=0. X-ray and Sunya
ev-Zeldovich effect observations should provide such a test.