  The recent discovery of a Higgs boson with mass of about 125 GeV, along with
its striking similarity to the prediction from the Standard Model, informs and
constrains many models of new physics. The Higgs mass exhausts one out of three
input parameters of the minimal, five-dimensional version of universal extra
dimension models, the other two parameters being the Kaluza-Klein (KK) scale
and the cut-off scale of the theory. The presence of KK fermions with large
coupling to the Higgs implies a short-lived electro-weak vacuum, unless the
cut-off scale is at most a few times higher than the KK mass scale, providing
an additional tight constraint to the theory parameter space. Here, we focus on
the lightest KK particle as a dark matter candidate, and investigate the
regions of parameter space where such particle has a thermal relic density in
accord with the cosmological dark matter density. We find the paradoxical
result that, for low enough cutoff scales consistent with vacuum stability,
larger than previously thought KK mass scales become preferred to explain the
dark matter abundance in the universe. We explain this phenomenon by
pinpointing the additional particles which, at such low cutoffs, become close
enough in mass to the dark matter candidate to coannihilate with it. We make
predictions for both collider and direct dark matter searches that might soon
close in on all viable theory parameter space.
