Sabine Hossenfelder does her usual spot on job of navigating through the weeds of what it means in science to say that something exists, using the Higgs boson, quarks, and gravitational waves as examples. An excerpt:
When we say that these experiments measured “gravitational waves emitted in a black hole merger”, we really mean that specific equations led to correct predictions.It is a similar story for the Higgs-boson and for quarks. The Higgs-boson and quarks are names that we have given to mathematical structures. In this case the structures are part of what is called the standard model of particle physics. We use this mathematics to make predictions. The predictions agree with measurements. That is what we mean when we say “quarks exist”: We mean that the predictions obtained with the hypothesis agrees with observations.
She goes on to discuss the philosophical concept of "realism" and to, appropriately, dismiss it as basically irrelevant.