Scientific theory is informed & peer reviewed conjecture; as long as theory holds, then fine but once it is overturned, out it goes. Take Dark Matter, of which it is believed 96% of the cosmos must be composed because, according to Newtonian Dynamics, it wouldn't work otherwise. No one has ever seen Dark Matter; it's existence is purely mathematical.

I was reading the other day about MOND (Modified Newtonian Dynamics) as postulated by Modehai Milgrom which could reproduce the rotation of galaxies without dark matter*. ESA are taking this seriously enough to launch a probe this autumn to put MOND to the test...

For more see WIKI : Modified Newtonian dynamics

* Bold type lifted from BBC Focus magazine 284, August 2015 p.82