This is a list of speculative, fringe or otherwise disregarded theories in science. (Note that theories with an extremely small base of followers are not encyclopedic and should not be included.)

Many of these theories are considered pathological science; this term refers to a psychological process in which believers in a theory, who may have originally conformed to the scientific method, unconsciously veer from that method, and begins a pathological process of wishful data interpretation. The article on pathological science provides more details on that subject. Historically, some current scientific conventions used today would appear on such a list (such as plate tectonics).

• The Cold fusion theory claims that hydrogen can undergo nuclear fusion at room temperature in palladium.

• The Duesberg hypothesis states that the HIV virus does not cause AIDS.

• Dynamic theory of gravity

• Hidden variable theory

• The Hollow Earth theory claims that the Earth is hollow, and is possibly populated by a race of superbeings, humans or aliens, and possibly dinosaurs.

• Luminiferous aether

• Orgone energy

• Plasma cosmology is a sub-set of plasma physics which attempts to explain large scale structure in the universe.

• Quaternion physics

• Reciprocal System of Theory postulates a universe of nothing but motion.

• The Steady state theory holds that the universe has always existed, and always will exist.

• Superficial anatomy

• Gene Ray's Time Cube. It is doubtful whether this concept even qualifies as a theory as it does not provide any testable constructs and Gene Ray refutes the scientific method itself.

• Pseudoscience
• Protoscience
• Occam's razor