ACT Science Assumption Questions

On the ACT Science Test, assumption questions will either ask you to apply a given set of information to a new situation or ask you to correctly identify the basis for a conclusion.

What is an assumption?

In an argument, an assumption is an underlying basis for a conclusion. It is something that needs to be true in order for a certain conclusion to be based on specific evidence.

An assumption always strengthens an argument or theory. On the ACT Science Test, the scientist may be stating a hypothesis based on data or evidence. The assumption is something that will "reinforce the implicit ideas" in the hypothesis, or reinforce the findings.


The velocity with which a small body A orbits a much more massive body B depends on the mass of body B and the distance between the two bodies: If body B is twice as massive, body A will orbit it at twice the velocity. In certain cases, however, the measured velocities of certain stars in a galaxy are greater than predicted based on their known mass.  This indicates that the galaxy must contain "dark matter," which is not visible.

Figure 1 shows how dark the presence of dark matter alters the velocities of the stars in a hypothetical galaxy: The lower "no dark matter" curve shows the theoretical velocities in a galaxy containing 100 million stars similar to the Sun in a sphere 5 kiloparsecs (kpc) in radius, and the upper "dark matter" curve shows the theoretical velocities if the stars are embedded in the center of a dark matter cloud that is much larger and much more massive than the visible galaxy.

Study 1

Astronomers have measured the velocities of stars in the galaxy NGC 3198.  The results are shown in Figure 2.  The "no dark matter" model in Figure 1 would have predicted a decline in the velocities at distances greater than the 5 kpc radius of the galaxy, but instead they remain approximately constant.

Study 2

Astronomers have measured the velocities of stars in 100 additional galaxies of approximately the same size (diameter and mass) as NGC 3198. The velocities of the stars are approximately the same as in NGC 3198.

Question 1: Which of the following statements correctly describes a possible reason why many additional galaxies were examined in Study 2?

  1. To determine whether they showed the same evidence of dark matter as NGC 3198.
  2. To determine the theoretical relationship between velocity and distance from the center of the galaxy.
  3. To confirm that the velocities of the stars in these galaxies also match the theoretical "no dark matter" predictions.
  4. To determine whether the same velocity changes occur in much larger galaxies.


We know this is an Assumption question because of the phrase "a possible reason". Think what is a logical basis for the set-up of Study 2? Notice how Figure 1 shows that the curves for the "no dark matter" and "dark matter" cases differ greatly.

This is evidence for the presence of dark matter in this particular galaxy, and a study of other galaxies with similar characteristics using the same technique could indicate whether dark matter is a typical feature of those galaxies. The answer is (A).