The statement that "reality doesn't exist until we measure it" is a popular interpretation of certain aspects of quantum mechanics, but it is not the only interpretation, nor is it universally accepted among physicists. It is associated with the Copenhagen interpretation, which is one of several interpretations of quantum mechanics.
According to the Copenhagen interpretation, the act of measurement in quantum mechanics collapses the wavefunction, determining the outcome of a specific observable. Prior to measurement, the system is described by a superposition of multiple possibilities, and it is only through measurement that one of these possibilities is realized. This interpretation suggests that the act of measurement plays a fundamental role in defining and determining the properties of a quantum system.
However, it is important to note that interpretations in quantum mechanics go beyond the empirical predictions of the theory itself. They involve philosophical and conceptual questions about the nature of reality and the relationship between the observer and the observed system. Different interpretations offer alternative perspectives and explanations for the same mathematical formalism.
Other interpretations, such as the Many-Worlds interpretation or the Pilot-wave theory (also known as Bohmian mechanics), propose different ways to understand the behavior of quantum systems without invoking the collapse of the wavefunction or the need for an external observer.
Ultimately, the question of whether "reality doesn't exist until we measure it" is a matter of interpretation and philosophical debate. Quantum mechanics is an extremely successful and empirically validated theory that provides accurate predictions for a wide range of phenomena. However, the interpretation of what these predictions mean in terms of the nature of reality remains an open question, and different physicists may have different perspectives on this matter.