What are the wires in this old switch box?
This is a pretty typical install
This is about what you'd expect to see in a situation where power is brought to the switch first, then carried to the light along with the switched (dimmed, in your case) hot for the fixture.  The wire junction with the white pigtail to the dimmer sticking out of it is that always-hot power I mentioned, while the reddish wire is the switched/dimmed hot going off to the chandelier, and the wire junction that does not connect to the dimmer is neutral.
So, your new dimmer should just be able to be wired up with the existing pigtail on one hot screw and the reddish wire on the other hot screw, provided it doesn't have any smarts, of course.  Grounding is done through the screws mounting the dimmer's yoke in this case, since the box is sort-of-grounded through metal cable armor from the looks of things.