Dynamical dark energy (DE) has been proposed to explain different aspects of the cosmological constant (CC) problem(s). For instance, within the standard LCDM model it is very difficult to accept that we just accidentally happen to live in an epoch where the CC contributes an energy density value right in the ballpark of the rapidly diluting matter density. To cure this and other problems one may entertain the possibility that the total DE is made out of two dynamical components of different nature: viz. a variable cosmological term, L, and another dynamical entity, the “cosmo” (denoted X), possibly interacting with L but not with matter – which remains conserved. We call this scenario the LXCDM model. One possibility for X would be a scalar field, but it is not the only one and in fact it could be totally unrelated to it. Both the dynamics of L and of X could be linked to high energy effects near the Planck scale.

We find that the cosmological evolution within the LXCDM model can meet a future stopping of the Universe expansion and can keep the ratio of the DE density to the matter-radiation density bounded and of order one. Therefore, the model could explain the so-called “cosmic coincidence problem”. We note that a cosmic halt could occur even if L > 0, because of the peculiar behavior of X as “Phantom Matter”. While the entity X can indeed behave phantom-like by itself, the overall DE fluid may effectively appear as standard quintessence, or even mimic at present an almost exact CC behavior (as observed). In this talk I will describe various cosmological scenarios made possible by the composite and dynamical nature of the LXCDM.