Reaction time is a stimulus response.

Complex reaction time is a stimulus response

Complex reaction time is the ability to respond to multiple situations at one time.  By "react" we mean seeing the danger, making a decision on what action to take to avoid a collision, and then taking the action based on that decision. Most of the time we use this terminology in driving terms. In broad sense the reaction time response is the theory of Psychology which says that it is the amount of time that it takes an individual to process information.

3. dark matter

empty space could also be a simple vacuum