Yes, I know exactly what you mean, and I've always pictured them as firm little globules of yellow fat, kind of like the fat that I find when I'm handling raw chicken. Because that's what I think they are. They're especially dense in areas that seem to be stubbornly hanging onto fat deposits, like my upper arms & the slight overhang just above my navel.
I've heard that fat gets like this during its last stand, before it starts dissipating. I sincerely wish that were so, but I don't know if that's "bro science" or a myth or what.