  We present a theory for the Kondo spin-1/2 effect in strongly correlated
quantum dots. The theory is applicable at any temperature and voltage. It is
based on a quadratic Keldysh effective action parameterized by a universal
function. We provide a general analytical form for the tunneling density of
states through this universal function for which we propose a simple
microscopic model. We apply our theory to the highly asymmetric Anderson model
with $U=\infty$ and describe its strong coupling limit, weak coupling limit and
crossover region within a single analytical expression. We compare our results
with numerical renormalization group in equilibrium and with a real-time
renormalization group out of equilibrium and show that the universal shapes of
the linear and differential conductance obtained in our theory and in these
theories are very close to each other in a wide range of temperatures and
voltages. In particular, as in the real-time renormalization group, we predict
that at the Kondo voltage the differential conductance is equal to 2/3 of its
maximum.
