  It is demonstrated that entropy and its density play a significant role in
solving the problem of the vacuum energy density (cosmological constant) of the
Universe and hence the dark energy problem. Taking this in mind, two most
popular models for dark energy - Holographic Dark Energy Model and Agegraphic
Dark Energy Model - are analyzed. It is shown that the fundamental quantities
in the first of these models may be expressed in terms of a new small
dimensionless parameter. It is revealed that this parameter is naturally
occurring in High Energy Gravitational Thermodynamics and Gravitational
Holography (UV-limit). On this basis the possibility of a new approach to the
problem of Quantum Gravity is discussed. Besides, the results obtained on the
uncertainty relation of the pair "cosmological constant - volume of
space-time", where the cosmological constant is a dynamic quantity, are
reconsidered and generalized up to the Generalized Uncertainty Relation.
