A natural phenomenon by which bros intrinsically congregate. Modern science describes brovity using the general theory of brolativity in which brovitation is a consequence of the curvature of the space/bro fabric.
The curvature of the space/bro fabric is attributed to fact that a single bro has the propensity to naturally bend his relative locus. The bending of the space/bro fabric is a result of bromass. Under these circumstances, bros exert an attraction on one another.
*Empire State of Mind plays...*
Passerby: Whoa... what's going down at the Tau Phi Alpha house?
Another Passerby: I don't know... It's a Thursday night and there has to be at least 70 bros over there. What's the deal?
Speculative Onlooker: It's brovity. The universal law by which bros naturally exert an attraction to other bros. Three bros throw a beer social and in little time they'll attract more and more bros... Natural law my friends.
Passerby: Looks like a complete sausage fest.