Gravity is the most clearly evidenced force around us, and yet it remains one of the biggest enigmas. The most brilliant mind of the past millennium was undoubtedly Isaac Newton. He discovered that gravity has the ability to influence action at a distance, that it is the force that attracts objects. Not only did he realize this, but he was able to express gravity’s influence on mass in mathematical terms. The most brilliant mind of the past century was Albert Einstein. He modified Newton’s calculations about gravitational pull when he realized that the curve in space-time bends light rays as they pass over a massive object.

Even with these monumental contributions, many questions remain. For almost a century, the world has awaited another Newton or Einstein to make sense of the fact that the vast majority (85 percent) of the gravitational force in the universe that we’ve managed to calculate is derived from entities that do not respond to the energy and matter we’re familiar with.

The Swiss-American physicist Fritz Zwicky was the first to bring up this problem of “missing mass” in the 1930s. He identified the odd phenomenon in the movement of galaxies in the Coma Bernices, an enormous cluster of galaxies 300 million light-years away from our own. Zwicky observed that galaxies were moving at high enough velocities to break away from their orbital circuits and go hurtling off into space—but they didn’t for some reason. If Earth were moving just 1.4 times faster than its current speed, it’d slip off the track, but these galaxies weren’t getting derailed from their courses: they stayed on their circuits, flying at speeds far beyond their “escape velocities,” which indicated that there was far more mass present than could be observed with the tools at science’s disposal.

There is a mysterious, invisible mass keeping the stars in their orbits. The common name that astrophysicists now employ for this is “dark matter.” The word “dark” does not indicate its appearance, but that it is something all together different than anything we’ve known. We are no closer to figuring it out because astrophysicists can’t analyze what they can’t observe.