No, they are not. Or, rather, they are only if they follow a(n inverted) chain line (i.e. exp(x)+exp(-x)), but the lower parts of the half spheres favored in architecture are steeper than that, so there is massive tensile stress there. This is, by the way, the genius in Wren's consruction of St. Pauls in London: he hid a chain line dome that carries most of the load between the visible outer and inner half spheres.

I was planning to mention arch/keystone before, but decided against it. One of the neat things about a DOME is, you can get the keystone effect without, y'know, a keystone. That ring of stone(s?) around the oculus fills the same compressive role, and lights the place up a bit in the bargain!

There is one issue that's NOT clearly answered just looking at the building or its design (not to me, anyway): an arch/dome pushes outward on its supports, not just downward, and requires some inward force/resistance to counter it. This is what the flying buttresses on many medieval cathedrals are for. I don't really see such a structure on the Pantheon, but I notice the walls are a bit thicker at the base. Whether that's enough to counter the expansive force, I can't say.

Clearly, something has kept it up there for two millennia.

More importantly, I don't think rebar has anything to do with durability (except insofar as being able to survive rare earthquakes). Rebar isn't impressive because it makes buildings last longer than without it, it's impressive because it allows building to be built which couldn't exist period without it.