  We aim to build a simple model of a gas with temperature ($T$) in thermal
equilibrium with a black-body that plays the role of the adiabatically
expanding universe, so that each particle of such a gas mimics a kind of
"particle" (quantum) of dark energy, which is inside a very small area of space
so-called Planck area ($l_p^{2}$), that is the minimum area of the whole
space-time represented by a huge spherical surface with area $4\pi r_u^2$,
$r_u$ being the Hubble radius. So we should realize that such spherical surface
is the surface of the black-body for representing the universe, whose
temperature ($T$) is related to an acceleration ($a$) of a proof particle that
experiences the own black-body radiation according to the Unruh effect. Thus,
by using this model, we derive the law of universal gravitation, which leads us
to understand the anti-gravity in the cosmological scenario and also estimate
the tiny order of magnitude of the cosmological constant in agreement with the
observational data.
