I constantly see various figures tossed around stating what the total mass contained in the universe should be versus what it is observed to be. Apparently, quite a bit of it is missing. Maybe it's because my exposure to this problem is through articles aimed at my level of understanding (the well-informed layman), but there seems to be an aspect of the problem that I've never seen explicitly addressed. Energy is equivalent to mass, and vice versa. Most matter in the universe radiates energy in the form of heat or light. The easiest example of this would be a star like our own sun. It has a finite life span during which it will emit light and heat and various other forms of EM radiation. The vast majority of this energy will never hit or interact with anything. It is simply sent out into the void. This didn't really bother me until I started thinking about the Cosmic Microwave Background radiation. The radiation we observe from this source is billions of years old. That energy has been in transit for billions and billions of years before reaching us, and most of it whizzes right on by. It's pretty faint, but when you consider that every conceivable point within the observable universe is capable of observing that very same energy, it seems like it would be a staggeringly huge amount. That's just "right now"; what about the effect over time? Would it be a stretch to say that the entire history of the universe exists as EM radiation in transit, aside from what is absorbed or interacted with directly by other matter? I guess my question boils down to this: Is the sum total of all energy radiated by the matter in the universe throughout history accounted for in the calculations that determine how much matter should be present?