139. Read the following passage, then answer the question below.

In the 1930s, the Swiss astronomer Fritz Zwicky studied the Coma Cluster, a group of galaxies bound together by gravity, and found a puzzle. The galaxies were moving so fast that the cluster should have flown apart, unless there was much more mass present than the visible stars and gas could account for. Zwicky called this unseen material dark matter, but most colleagues paid little attention. Decades later, Vera Rubin and her collaborators measured how quickly stars orbit the centers of spiral galaxies. If most of a galaxy's mass were in its bright center, stars farther out should orbit more slowly, just as distant planets move more slowly around the Sun. Instead, stars on the outskirts moved just as fast as those near the center, suggesting that each galaxy is enveloped in a vast halo of invisible matter.

Further evidence came from gravitational lensing, in which the gravity of a massive object bends the path of light from objects behind it. The amount of bending observed around galaxy clusters exceeds what the visible matter could cause. Today, cosmologists estimate that dark matter makes up about eighty-five percent of all matter in the universe, although the exact figure depends on the model.

What dark matter is remains unknown. Candidates include weakly interacting massive particles, or WIMPs, and extremely light particles called axions, but experiments in underground laboratories have not yet detected either. A minority of physicists propose instead that the law of gravity itself changes at very large scales, a hypothesis known as modified Newtonian dynamics. It explains the rotation of galaxies well but struggles with observations of colliding galaxy clusters. Most researchers therefore consider particles the more likely explanation, while acknowledging that the question is still open. The search shows how science can proceed with confidence about an effect while remaining uncertain about its cause.

The word 'It' in paragraph 3 refers to