Quantitative research is a formal, objective, systematic process in which numerical data are used to obtain information about the world. This research method is used: to describe variables; to examine relationships among variables; to determine cause-and-effect interactions between variables

A target audience is the intended audience or readership of a publication, advertisement, or other message. In marketing and advertising, it is a specific group of people within the target market at which a product or the marketing message of a product is aimed

Niche" is a fraction of a total market or an audience. A segment of the market or the audience with highly specific needs which cannot generally be satisfied by many service providers or marketers. for exmaple, people who like watching films about modern dystopias.

Mainstream is the common current thought of the majority. 
So a mainstream audience is the majority of people who go to the theatre that share the same opinion. 
For recent Superhero films, e.g. Avengers, Man of Steel and Watchmen I would say that the main stream audience children to middle age people, and also those who read comic books.

Audience research is defined as any communicationresearch that is conducted onspecific audiencesegments to gather information about their attitudes, knowledge,interests, preferences, or behaviours with respect to prevention issues.

A group of people assembled to participate in a discussion about a product before it is launched, or to provide feedback on a political campaign, television series, etc.