The phrase "extensive research" refers to a thorough and exhaustive investigation or exploration of a particular topic, issue, or problem. It involves gathering information from various sources such as books, journals, articles, reports, interviews, surveys, and experiments, among others. The goal is to collect enough data that can help provide answers, solutions, or insights into the subject being studied. Extensive research requires a lot of time, effort, and resources, but it yields valuable information that can be used to make informed decisions, develop new theories, or solve complex problems.