Which type of system does NOT allow matter to flow into or out of it?

A closed system is defined by its inability to allow matter to enter or leave. Instead, it can exchange energy with its surroundings, but the total amount of matter within the system remains constant. This concept is foundational in various scientific fields, such as thermodynamics, where understanding energy conservation and material balance is critical.

In contrast, an open system allows both matter and energy to flow in and out freely, which would not meet the requirement of the question. Dynamic systems typically involve interactions that can change over time but do not specifically restrict matter. Fluid systems involve the movement and behavior of liquids and gases, which also entails the exchange of matter. Thus, the characteristics of a closed system clearly align with the stipulation of not permitting any matter to flow in or out.