Question 40·Easy·Text Structure and Purpose
For decades, astronomer Vera Rubin collected data on the way stars move around the centers of their galaxies. She noticed that, contrary to what Newtonian physics predicted, stars at a galaxy’s outer edge revolved just as quickly as those near the center. Rubin’s careful observations provided some of the first compelling evidence for the existence of “dark matter,” an invisible substance thought to make up most of the universe’s mass.
Which choice best states the main purpose of the text?
For main-purpose questions on short SAT passages, first read the entire passage and then summarize it in one simple sentence: "This passage is mainly doing X." Next, scan each answer choice and cross out any that mention content or goals you didn’t see (like detailed math, a debate, or a comparison). Also eliminate choices that contradict key ideas (for example, saying the passage argues against something it clearly supports). Choose the option that best matches your one-sentence summary of what the passage as a whole is doing, not just a single detail or phrase.
Hints
Restate the passage in one simple sentence
Before looking closely at the choices, try to say in your own words what the entire passage is mainly doing from beginning to end.
Watch out for what’s not in the text
Ask yourself: does the passage actually show calculations, make an argument for or against something, or compare people? Or does it mostly report what one scientist found and why it mattered?
Eliminate choices that contradict key details
Look for any choice that says the text argues the opposite of what is described, or that mentions types of content (like detailed math or comparisons) that you never saw in the passage.
Step-by-step Explanation
Identify the question type
The question asks for the main purpose of the text. That means you must decide why this short passage was written overall, not what any one sentence says in isolation.
Summarize the passage in your own words
Put the passage into a simple one-sentence summary: it introduces astronomer Vera Rubin, explains what she spent decades measuring about star motion in galaxies, notes a surprising pattern that didn’t match Newtonian expectations, and states that this became important evidence for something called dark matter.
Notice what the passage does not do
The passage does not show any actual equations or calculations from Newtonian physics, it does not argue against dark matter, and it does not mention or compare any other astronomers. It is focused on Rubin’s work and what she found.
Match that summary to the answer choices
Now compare your summary to the choices:
- A talks about detailing the mathematics of Newtonian physics (which the passage doesn’t do).
- B says the text argues against dark matter’s influence (the passage actually presents evidence for dark matter).
- D talks about comparing Rubin to other astronomers (no comparison appears).
- C accurately reflects that the passage explains what Rubin observed and how those observations supported the idea of dark matter.
Therefore, the best answer is: To describe Rubin’s observations that supported the concept of dark matter.