As far as I know, it's completely unrelated to dark matter so this discovery doesn't say anything about that.

The main point of interest is that, in QCD, bound states are very difficult to describe theoretically. So it's easy to play around with ideas, but not so easy to make an actual prediction. More often than not, making a prediction in QCD can't be done from first principles alone, you have to make approximations, assumptions and modelling choices that lower the strength of that prediction.

Without going into too much details: in quantum field theory, degrees of freedom get redefined as the scale of observation (energy) changes. So an "isolated" gluon (high energy) and a gluon inside a bound state like a glueball (low energy) might have very different behaviors that are extremely difficult to connect.

So having a proof of existence for a given bound state both gives more confidence to the choices made to predict an important parameter like its mass, and strengthens this sector of QCD.

Glueballs are actually one of the 'easiest' things to simulate in lattice QCD - so easy that it was possible to start getting results for them back in the 80s.

https://www.sciencedirect.com/science/article/abs/pii/0370269382912813

And by the 90s it was a suitable project to give a bunch of over eager 3rd year undergraduates who wanted to do a final year lab project that didn't require any actual lab.

Shout out to the wonderful Dr Ray Rivers for that one!