[单项选择]
阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案。

Dark Forces Dominate Universe The Earth, moon, sun and all visible stars in the sky make up less than one percent of the universe. Almost all the rest is dark matter and dark energy, unknown (1) that puzzle astronomers. Observations in recent years have changed the basic understanding of how the universe evolved and have emphasized for astronomers how little is known (2) the major forces and substances that shaped our world. Astronomers now know that luminous matter-stars, planets and hot gas — (3) only about 0.4 percent of the universe. Non. luminous components, such as black holes and intergalactic gas, make up 3.6 percent. The rest is either dark matter, about 23 percent, or dark energy, about 73 percent. Dark matter, sometimes called "cold dark matter" has been known for some (4) . Only recently have researchers come to understand the pivotal