IIUC, it's taken seriously as a mathematical exercise but it absolutely relies on phenomena we've never observed in nature (i.e. matter with a behavior akin to negative gravitation).

There is a possibility antimatter could fit the bill for the "exotic matter" required in the model (we've never accumulated enough of the stuff and held it in a controlled environment long enough to do any tests on how gravity interacts with it), but I wouldn't bet money on that outcome yet.