Dark Chemical Matter (DCM) in screening deck

I was interested in the article, because of the title. “The performance of dark chemical matter in high throughput screening” https://www.ncbi.nlm.nih.gov/pubmed/27762554 What is dark chemical matter(DCM) ? The definition of DCM is that compounds, which have been tested and found inactive in 50 or more assays, exhibit hit rates that are comparable to those of