Why are black holes considered to be undetectable?

Black holes are considered undetectable because they emit no light and are therefore invisible. This characteristic is central to their nature; a black hole's gravitational pull is so strong that not even light can escape from it once it crosses the event horizon. As a result, they cannot be observed directly in the traditional sense using light-based instruments. Instead, the presence of black holes is inferred through their effects on surrounding matter, such as stars orbiting around an unseen object or radiation emitted from material as it falls into the black hole.

While other options touch on aspects related to black holes, they do not address the fundamental reason for their undetectability as effectively as the correct choice. For instance, while it is true that black holes were initially a theoretical concept, many have been confirmed through observational evidence. Similarly, their size and location can vary, but these factors do not fundamentally explain why they are undetectable. The essence lies in their inability to emit light, rendering them invisible against the vast backdrop of space.