Final answer:
Dark matter is widely accepted because of compelling indirect evidence supporting its existence, unlike UFOs, which lack scientific backing. Scientists would gain confidence in dark matter through direct detection of exotic subatomic particles or irreconcilable gravitational phenomena.
Step-by-step explanation:
The question revolves around the belief in dark matter versus the skepticism towards UFOs as evidence of alien visitation. Astronomers' widespread acceptance of dark matter's existence, despite having never seen it, is due to indirect evidence that supports its presence in the universe. This evidence includes gravitational effects on visible matter, light, and the cosmic microwave background, which do not align with our current understanding of gravity and visible matter alone. In contrast, the concept of UFOs as evidence of alien life is not widely accepted because it lacks rigorous, scientific evidence and is often associated with unverified or anecdotal reports.
Regarding the observations that could increase confidence in the existence of dark matter, hypothetical scenarios could include the direct detection of exotic subatomic particles through sophisticated experiments, which are suggested as the components of dark matter. Alternatively, consistent observations of gravitational lensing events that cannot be accounted for by visible matter could reinforce the theory of dark matter. In essence, concrete evidence that firmly cannot be explained by current understanding of known particles and forces would bolster the case for dark matter's existence.
A historical example similar to dark matter's current status is the initial confusion caused by the discovery of Uranus, which was explained and became a well-understood part of astronomical textbooks after more observations and the development of better theories.