80 years ago scientists discovered the neutron. The masses of the two nucleons, the neutron and the proton, are so close that in middle school it doesn’t make sense to distinguish the difference. Last week scientists have determined that the neutron is ≈0.14% more massive than the proton. This implies that the down quark is slightly more massive than the up quark.
If the difference is masses were slightly different, our universe would be very different, with not as much hydrogen, not many heavy metals, or more.