If you can't find an encyclopedia, dictionary, or textbook article on your topic, think broad.
If your topic is global warming for example, search for an encyclopedia on the environment. Once you locate the environmental encyclopedia, look in the index for global warming.
You will always be able to find background information on your topic--usually within an encyclopedia on a slightly broader subject/topic.
Image source: Man vyi. Public domain. Wikimedia Commons.
Background research serves many purposes. It will usually:
Background information can be found in:
Image Source: Marcus Cyron. CC BY-SA 3.0. Wikimedia Commons.