Someone's calculations based on certain assumptions says that there is missing mass and missing energy in the universe. These unfound properties of the universe go under the names of Dark Energy and Dark Mater. Finding dark mater is actually easy to do. We know we can't easily see non reflective objects with our telescopes at a distance when they are also not self illuminated by any wavelength. We find possible planets just by observing wobbles of distant starts and make assumptions that something unseen is planetary. We know that asteroids and comets in our own solar system are dark sooth black encrusted objects that reflect less light than we might have expected. We fantasize that comets are bright white ice balls but reality shows with up close views that they are black body mater until they develop a coma in proximity of the sun. Same is true of asteroids and speculations is that if Pluto was not an active planet it too would be darker in color than it's surface appears. Dark mater is easy to accounted for and so is Dark energy.
If there is dark energy in the universe we should already know what it is. A massive object gains velocity relative to other objects in space and if it becomes a projectile that can hit another such object in space their collision reveals a lot of energy that is not otherwise immediately visible. They can be dark cold objects at absolute zero temperature and if traveling at hundreds of thousands of meters per second and colliding at double that speed they can vaporize each other because they have so much unseen energy. A car going 70 mph on the freeway in collision with one coming in the opposite direction going 34 mph crashes with the energy of 104 miles per hour force. It is not very pretty a crash like that. Head on its not fast enough to vaporize either car but it does release unseen in felt energy. To stop a car going 70 mph the brakes need to be applied to drain the car and everything and everyone in it of that dark energy. The energy cost of breaking is a significant amount of fuel . People riding in a car at 70 mph might as well feel as if they are sitting still in their seats. If the car was perfectly quiet and the windows covered with no vibrations the passengers might think they were sitting still except maybe during acceleration and deceleration ... Dark Energy is everywhere were everything is moving.
When we relate dark energy defined as the momentum of mass with velocity and we judge distances in astronomy with the speed of light than we actually start to re-equate our notions. Clearly if light across the EM spectrum has anything to do with definitions and assessments of dark energy than we might want to reconsider what we think light is and is not. Light if having a known set velocity then would have to be interacting with mass as it does not by transmitting energy though space but by tapping into those invisible reserves of dark energy of motion. It sounds astounding and unbelievable until you start looking the actual phenomena of light and how it interacts Light will be found to be just a electro magnetic code sent for short range purposes in matter itself transmitting no actual energy and actually finding and utilizing store of energy from that momentum . effectively showing down sub atomic particles relative to the rest of the object. The way to understand it is the collision of sub atomic particles of atoms with photons. photons also carry no mass, just an analogy of mass, The collision then between mass and energy is what is important. light at it's phenomenal. speed of light being a constant than makes some more sense in collisions.