Light is an energy form. There’s a bunch of them, like electricity, magnetism, radioactivity, gravity and radio, aka electromagnetic waves. For decades now, scientists and mathematicians have been trying to find the common denominator in all the different forms of energy, in a Unified Field Theory. But so far, no luck. If they were successful it would open up the manipulation of gravity, just for starters.
Light is weird. It passes right through many things, including most gases and any form of clear glass, though different types of glass affect light differently. At the same time, light has mass.
Because light has mass, it can push on things. Hence the plans for space ships powered by light sails, for instance. Keep that one in mind because we’ll be coming back to it.
Light has frequencies, like radio waves, and that’s what makes colors. Our eyes can see the different frequencies and our brains interpret that as colors. Now, radio waves can be weak and not carry far, or strong and carry to the limits of our solar system and beyond. Light waves, (as in frequencies) can also be strong. Very strong. We have lasers now that can cut through metal, and lasers are just light. Only lots of it in a very tiny beam.
Now lets seemingly digress just a sec, and consider that our Universe is known to be not only expanding, but doing so at a steadily increasing rate. Science has been seeking the reason for this ever since it was discovered, and theories of Dark Matter that no one can see or test for have arisen, and Negative Gravity, and all sorts of other arcane theories have been proposed.
So in this spirit, I propose the following: Our Universe is filled with stars. Stars give off one hell of a lot of light. When the light from one galaxy encounters another, it pushes it. In fact, all the galaxies are pushing against each other with their light output, and if you push long enough (will 13 billion years do it? the galaxies should all start moving away from each other, just like raisins in expanding bread dough, and exactly how the Universe is in fact expanding.
The increasing rate of expansion is what a light sail is supposed to do, go faster over time, and what our Universe is also doing.
Occam’s Razor is the well-known principle that the simplest explanation is usually the correct one.