Could the inferred existence of dark matter and dark energy be due to a modified behavior of gravity?
It’s possible, and in fact there are scientists working hard on just this scenario — doing away with dark matter and/or dark energy, and instead invoking a new law of gravity on very large scales. There are a few obstacles to this idea, though, and two are worth stressing. One is that the dark matter/dark energy paradigm does an extremely good job of explaining the data, and in a wide variety of apparently disconnected circumstances. The other is that our current theory of gravity — Einstein’s general theory of relativity — is both conceptually compelling and experimentally very well tested. It’s hard to come up with a new theory that fits the data nearly as well as the conventional model of dark matter and dark energy in the framework of general relativity (but that’s no reason not to keep trying).
