My coworker, who is newer than me, with the same position title and same exact work tickets is getting paid 30% more than me. What should I do?

For extra context, we both internally transferred from within the company. I did have a lower rate at my previous position. I’m sure I was lowballed because of that. My boss will not adjust my compensation to the market (I already asked). I found out after we talked about each others salary and I was internally furious but I haven’t brought any of this up at work. I also worked at the position half a year longer. We have the exact same work and same title.