how can you understand something from its absence?

Answer 1

It's hard to tell exactly what you mean without more information. But there are a lot of ways in which lack of information can shed light on a topic. For example, it was at one time believed that there was a planet orbiting closer to the Sun than Mercury, because irregularities were detected in Mercury's orbit. No such planet was found, and this eventually led to refinement of relativity to show that the Sun's gravity was warping space to a great enough extent to have the perceived effect on Mercury. When data to support a widely accepted hypothesis fails to materialize, it can spur the scientific community to examine the issue from other perspectives.