What can be observed only by observing its effects on gravity.1 dark matter 2.solar nebula 3.gas and dust 4.planetesimal

1. dark matter

Explanation:

Majority of the universe is composed of dark matter (about 85%). This cannot be observed and scientists are not sure what makes the dark matter. But its effect on gravity can be observed. Without dark matter, the galaxy could not be held together. The velocities of stars is different in different parts of the galaxy. Thus, Dark matter can only be observed by observing its effect on gravity.

Dark matter

Explanation:

Dark matter

Unlike normal matter, electromagnetic force doesn't interfere with dark matter. It doesn't absorb, transmit or emit heat, which makes it extremely difficult to spot. In fact, scientists were only able to deduce the existence of dark matter from gravitational effect that it tends to be on visible matter.