Grey literature is the unpublished, non-commercial, hard-to-find information that organizations such as professional associations, research institutes, think tanks, and government departments produce. It can be invaluable to your review: it is part of the overall evidence base and is an alternative resource that may be used to overcome possible bias presented by published information.
The Grey Literature research guide provides a number of resources that are useful additions to systematic reviews, including ClinicalTrials.gov, European Clinical Trials Register, NIH RePORTER, & OAIster, as well as links to sources for dissertations & conference proceedings.
Unpublished data can be hard to locate; this article can provide helpful suggestions: Young T, Hopewell S. Methods for obtaining unpublished data. Cochrane Database Syst Rev. 2011 Nov 9;(11). PMID: 22071866.