The Copenhagen interpretation is that the act of observation has an effect on the observed. Specifically, this theory states that a subatomic particle is not in a definite physical state until it is observed, and that the act of observation causes the subatomic particle to "collapse" into a definite state for the observer. If the observation does not occur, then the subatomic particle continues to behave like a wave rather than like a particle.
Materialists resist accepting the Copenhagen interpretation, instead insisting that there is an underlying reality that does not depend on observation. These opponents of the Copenhagen interpretation propose or insist upon the existence of local hidden variables that are not yet discovered. Yet every single test of the Copenhagen interpretation, including a series of Bell test experiments, has confirmed its truth.