Question

what can be observed only by observing its effects on gravity? dark matter a solar nebula gas and dust a planetesimal

Explanation:

Dark matter does not emit light or other electromagnetic radiation, so it cannot be directly observed. However, its gravitational effects on visible matter (such as stars and galaxies) can be detected. A solar nebula is a cloud of gas and dust from which a solar system forms and can be studied through various observational methods related to light and radiation. Gas and dust can also be observed through electromagnetic radiation (e.g., infrared for dust). A planetesimal is a small body that forms during the early stages of planet formation and can be studied through its gravitational effects on other bodies as well as through indirect observational techniques related to light (if it reflects or emits some). But the key characteristic here is that dark matter is primarily known and studied via its gravitational effects as it does not interact with light in a way that allows direct visual - like observation.

Answer:

dark matter