The HED domain is often defined by an energy density (units of pressure) above 1 Mbar = 100 GPa ~ 1 Million of Atmosphere. This is comparable to the energy density of a chemical bond such as in a water molecule. Thus at 1 Mbar, chemistry as we know it changes. Experiments at NIF now routinely probe matter at 100 Mbar. At these "atomic pressure" conditions the energy density is comparable to that of the inner core electrons, so the atoms themselves change. The dense HED regime includes highly degenerate matter, with interatomic spacing less than the de Broglie wavelength. This is similar to quantum regime achieved at low temperatures (e.g. Bose–Einstein condensation), however, unlike the low temperature analog, this HED regime simultaneously probes interatomic separations less than the Bohr radius. This opens an entirely new quantum mechanical domain, where core electrons - not just valence electrons - determine material properties and gives rise to core-electron-chemistry and a new structural complexity in solids. Potential exotic electronic, mechanical, and structural behavior of such matter include room temperature superconductivity, high-density electrides, first order fluid-fluid transitions, and new insulator-metal transitions. Such matter is likely quite common throughout the universe, existing in the more than 1000 recently discovered exoplanets.