Its characterizing bulk behaviour is that dark matter exerts a pressure on galaxies immersed in it, effectively squashing them inwards (or, if you like, preventing discoid galaxies from flying apart as they spin quickly).
Energy-density is encoded in the (symmetric, gauge-invariant, conserved, (0,2)) matter tensor of the Einstein Field Equations of General Relativity.
Among other things this preserves notions that matter/energy cannot be created or destroyed, only converted from one form to another.
It is the microscopic constitution of the bulk energy-density, and the details about how or if it can be converted into photons or some other radiation, or other types of matter, that is not known, although the bulk behaviour imposes numerous constraints.
As with all matter in General Relativity, dark matter is subject to the strong equivalence principle, just like laboratory Cavendish experiments, or binary or triple pulsar systems, or in galaxies' peculiar motions within galaxy clusters.
Finally, your "as simple that gravity Works Differently at galactic scales" is a rejection of the Strong Equivalence Principle, and also raises the question of the (unknown) microscopic details of how the inverse square law holds up so well everywhere except the edges of many known galaxies. Typical approaches involve a new long-range ("fifth") force that in bulk effectively applies a tension on the outer reaches of galaxies where MOND transitions from the familiar 1/r^2 inverse square law for gravity to the slower-decaying 1/r law. Tension has the dimensions of negative pressure, so essentially rather than outside-the-galaxy dark matter pushing the outer gas clouds and stars inwards, we have something inside the galaxy reaching out and pulling the same gas clouds and stars inwards. One is then left struggling to do anything other than to describe the microscopic behaviours of a field that sources this negative pressure, and making that field denser somewhere towards the middle of galaxies.
Its characterizing bulk behaviour is that dark matter exerts a pressure on galaxies immersed in it, effectively squashing them inwards (or, if you like, preventing discoid galaxies from flying apart as they spin quickly).
Under <https://en.wikipedia.org/wiki/Dimensional_analysis>, pressure has dimension L^{-1}MT^{-2}, which is the same as energy-density.
Energy-density is encoded in the (symmetric, gauge-invariant, conserved, (0,2)) matter tensor of the Einstein Field Equations of General Relativity.
Among other things this preserves notions that matter/energy cannot be created or destroyed, only converted from one form to another.
It is the microscopic constitution of the bulk energy-density, and the details about how or if it can be converted into photons or some other radiation, or other types of matter, that is not known, although the bulk behaviour imposes numerous constraints.
As with all matter in General Relativity, dark matter is subject to the strong equivalence principle, just like laboratory Cavendish experiments, or binary or triple pulsar systems, or in galaxies' peculiar motions within galaxy clusters.
Finally, your "as simple that gravity Works Differently at galactic scales" is a rejection of the Strong Equivalence Principle, and also raises the question of the (unknown) microscopic details of how the inverse square law holds up so well everywhere except the edges of many known galaxies. Typical approaches involve a new long-range ("fifth") force that in bulk effectively applies a tension on the outer reaches of galaxies where MOND transitions from the familiar 1/r^2 inverse square law for gravity to the slower-decaying 1/r law. Tension has the dimensions of negative pressure, so essentially rather than outside-the-galaxy dark matter pushing the outer gas clouds and stars inwards, we have something inside the galaxy reaching out and pulling the same gas clouds and stars inwards. One is then left struggling to do anything other than to describe the microscopic behaviours of a field that sources this negative pressure, and making that field denser somewhere towards the middle of galaxies.