I'm thinking about an explanation of what dark matter is. Would it make sense to say that energy has gravity? For example when a galaxy was formed after the Big Bang a big amount of energy was converted into matter but an even greater amount of energy could have never been converted into mass so it ends up surrounding the galaxy. So if all that energy has gravity wouldn't that stick to the description of dark matter? (surrounds galaxies, it's invisible and has a gravitational effect on the galaxy). You could think of energy as potential or virtual matter, where certain amount of energy floating in the space represents a certain amount of mass (that is already proven to be made of energy). It's like a dismantled atom but in certain conditions an spectator could feel its effect as a real atom.