The phrase "gray matter" refers to the brain tissue that is responsible for thinking, learning, and processing information. It is called gray matter because it appears gray in color when observed closely. Full definition
In this older study a different research group found changes in gray matter in brain regions of Internet addicts. (scientificamerican.com)
Better yet, there's plenty of evidence that shows a decrease of gray matter in the parts of the brain that are associated with fear and anxiety. (mindbodygreen.com)
Comparison images taken and two and four weeks postpartum revealed a small but significant increase in gray matter volume in specific areas of the brain. (verywellfamily.com)