Since Zwicky (1933), we have known that clusters of galaxies have gravitational potentials which are too large to be explained by the amount of visible baryons under the assumption of a Newtonian gravitational force law. This has led to competing hypotheses that either the masses of clusters are dominated by a non-baryonic form of matter or that gravity departs from a 1/r^2 force law on cluster scales. By using merging clusters of galaxies, I will show that the different types of matter in the clusters can be spatially seperated and, by using gravitational lensing, I will prove, independent of any assumptions about the nature of the law of gravity, that the dominant mass component of the clusters is not the visible baryons. I will also discuss how these observations can be used to place constraints on the nature of the dark matter, including a limitation on the self-interaction cross-section of any dark matter particles, and how these may be used to test alternative gravity models. I will also discuss how using the variation gravitational lensing strength with the redshift of the background galaxy being lensed can place constraints on dark energy models independent of any assumption of a standard candle or rod.