The statement that "nothing is real until it is observed" is a popular interpretation of quantum mechanics, often associated with the Copenhagen interpretation. However, it is important to note that this interpretation is just one way of understanding quantum mechanics and is subject to different interpretations and perspectives.
In the Copenhagen interpretation, which was developed by Niels Bohr and his colleagues, the act of measurement or observation is considered to be a fundamental part of quantum mechanics. According to this view, the quantum system exists in a superposition of possible states until it is measured, at which point the superposition collapses into a definite state. In this sense, the act of observation is seen as playing a crucial role in determining the "reality" of the system.
However, it's worth noting that this interpretation is not universally accepted, and other interpretations exist that offer different explanations for the behavior of quantum systems. For instance, the Many-Worlds interpretation suggests that all possible outcomes of a measurement occur in different branches of a continually branching multiverse. Other interpretations, such as the de Broglie-Bohm theory (pilot-wave interpretation) and the ensemble interpretation, offer deterministic or statistical descriptions of quantum systems without emphasizing the role of observation in collapsing the wavefunction.
Ultimately, the question of the nature of reality and the role of observation in quantum mechanics remains a topic of ongoing debate and philosophical inquiry. Different interpretations offer different perspectives on how to understand and interpret the mathematical formalism of quantum mechanics, and no consensus has been reached among physicists regarding the true nature of quantum reality.