Practice Tests
IELTS Reading (AC) Task 1 #181

A. Dark matter is a mysterious, invisible substance that is thought to make up approximately 27% of the mass and energy of the universe. Its existence was first inferred in the 1930s by astronomers who observed that galaxies were rotating much faster than they should be based on the amount of visible matter—stars, gas, and dust—they contained. This discrepancy implied that there must be a huge amount of unseen mass providing the extra gravitational pull needed to hold the galaxies together.

B. Despite decades of research, the nature of dark matter remains one of the biggest unsolved mysteries in modern physics. We know it exists because of its gravitational effects on visible matter, on the large-scale structure of the universe, and on the light from distant objects, a phenomenon known as gravitational lensing. However, dark matter does not appear to interact with light or any other form of electromagnetic radiation, which is why it is "dark" and cannot be observed directly with telescopes.

C. Scientists have proposed several candidates for what dark matter could be. The leading hypothesis is that it is composed of a new type of subatomic particle, not yet discovered, that interacts very weakly with normal matter. These hypothetical particles are often referred to as WIMPs (Weakly Interacting Massive Particles). Huge, highly sensitive experiments have been built deep underground, shielded from cosmic rays, in an attempt to detect the rare collision of a WIMP with an atomic nucleus, but so far, no definitive detection has been made.

D. Understanding the nature of dark matter is a fundamental goal of particle physics and cosmology. It is a crucial component of the Standard Model of Cosmology, which describes the composition and evolution of the universe. Unlocking its secrets would not only solve a major cosmic puzzle but could also lead to a new understanding of the fundamental laws of physics beyond the current Standard Model of Particle Physics.