Dark matter is a theoretical form of matter proposed to explain what phenomenon?

Dark matter is a theoretical form of matter proposed to explain what phenomenon?

• A. The motion of galaxies
• B. The color of stars
• C. The temperature of the Sun
• D. The brightness of nebulae

Answer: (A)

Understanding why galaxies rotate the way they do is the key idea. Observations show that stars and gas in galaxies orbit the center at roughly the same speeds even far from the center, where there isn’t much visible matter to account for the gravitational pull. If only the visible stars and gas contributed gravity, those outer speeds would be much lower and the outer stars would tend to fly away. Dark matter is proposed because it provides the extra mass that exerts the necessary gravity without emitting light, making the observed rotation possible. The other options relate to intrinsic properties like temperature or emission, not to unseen mass influencing galactic motion, so they don’t explain these rotation observations.

Understanding why galaxies rotate the way they do is the key idea. Observations show that stars and gas in galaxies orbit the center at roughly the same speeds even far from the center, where there isn’t much visible matter to account for the gravitational pull. If only the visible stars and gas contributed gravity, those outer speeds would be much lower and the outer stars would tend to fly away. Dark matter is proposed because it provides the extra mass that exerts the necessary gravity without emitting light, making the observed rotation possible. The other options relate to intrinsic properties like temperature or emission, not to unseen mass influencing galactic motion, so they don’t explain these rotation observations.