The idea that "nothing exists until it is viewed or measured" is a misunderstanding or misinterpretation of quantum theory. Quantum theory does not imply that the act of observation or measurement is responsible for bringing things into existence.
In quantum mechanics, the wavefunction describes the state of a system, and it evolves according to the Schrödinger equation, which is a deterministic equation. The wavefunction includes information about all possible states of the system, even if they are not currently observed or measured.
When a measurement is made, the wavefunction of the system undergoes a process called wavefunction collapse or projection, where it "collapses" into one of the possible eigenstates of the measured observable. The specific outcome is probabilistic and determined by the probabilities encoded in the wavefunction.
However, this does not mean that things only exist when they are observed. It simply means that the act of measurement determines which state of the system is revealed or observed by an observer. The unobserved states still exist in a superposition and can potentially be observed or measured in the future.
As for our existence, quantum mechanics is not relevant at macroscopic scales, such as the existence of everyday objects or living beings. Quantum effects become important at microscopic scales, such as the behavior of atoms and subatomic particles. Our existence and the macroscopic world are governed by the principles of classical physics, which are derived from the underlying quantum mechanical laws but are approximate descriptions valid under certain conditions.
So, our existence is not dependent on being observed or measured, and the principles of quantum mechanics do not imply otherwise.