What makes music heavy exactly?

Cheeseburgers.

To me it's just when music has a feeling of almost physical weight. Black Sabbath is heavy. Electric Wizard is heavy. Pantera is heavy. I generally don't think of Slayer in terms of heaviness, since speed can often detract from it. When they slow down, such as on "Skeletons of Society", or during the mid-paced breakdown on "Angel of Death", then they're heavy, but not so much when they're going Mach ten. If Dave Lombardo adds heaviness, it's in spite of the rest of the band.

Unpopular music opinions? Umm I would say mainstream music completely died around 1997, britpop was its swansong.

Does it have to have drums at all?
I've often heard people call drone doom the heaviest.

Another Pink Floyd unpopular opinion: the live half of Ummagumma is their best work.

Not necessarily an opinion, but an observation that may be an unpopular/uncommon trend: As I grow older, my musical interests are veering more closely to Contemporary Pop and less serious music. I prefer catchy music in general. Sure, there are certain artists that I like who are making weird avant-garde (for ecample) records, but I am pretty much over listening to music that require much attention.

I am the opposite. The more people rave about a band the more I want to avoid them.

Mainstream music still exists believe it or not.

I second this. Synchronization of drums and bass in thudding concert with one another has always sounded 'heavier' to me then speed riffing and cymbal smashing.

No problem with the latter, by the way.