How Do We Know That Light Has No Mass?

How would the universe be different if light had mass? And I guess specifically, how would it have developed differently? And how would it be different for us today if it had mass? Yes, this is a super fun question. It led me down a little rabbit hole of reading thinking about why is it any way that the photon has no mass? And do we actually know that the photonhas no mass? What? Okay, first of all, most people think the photon or light has no mass. That's right. And in our current theory, it has no mass, like the standard model of particle physics assigns it to be exactly zero mass. You can also go out in the universe