You raise two issues. Solipsism, the question of whether any other mind truly has subjectivity, or only appears to given that you can't directly verify like you do your own. And the consistency of unperceived phenomena.
Solipsism is a classic question for philosophy. Descartes approach was to apply successive degrees of skepticism, at the most extreme recognising only that we can be sure we are thinking, because contemplating the question in any way verifies that, to noting the clear and distinct quality of this understanding is a kind of evidence that is itself a useful criteria. And then building a hierarchy of knowledge from there. In modern philosophy philosophical zombies are used as a thought experiment to understand internal experience and qualia and whether they are essential for minds, or instead are epiphenomena. So it useful to consider solipsism, and philosophical work can be done from considering it. But someone truly embracing solipsism as a stance shouldn't be interested in other people's views, and so can't meaningfully engage with philosophy. There is more evidence for an external world, than against it, as well as practical arguments that it is useful, and beneficial to gene replication to believe in it - and our conditioning by genes is a crucial part of explaining how we can ask questions at all.
Unperceived phenomena are directly addressed by the thought experiment of whether a tree falling with no one to hear makes a sound. I would describe the physics perspective as based on conservation laws, and continuous symmetries which are proven equivalent. Known in enough detail, the future states of things predicts, or constrains, outcomes. It would be non-sensical to describe what seem to be universal laws describing the cosmos, as being founded in a local perspective. They cannot be altered by a persons mental activity, so they are external, and define the consistencies we call the external world. We can use complex systems on the edge of chaotic behaviour, and apply statistics to the outcomes, to show they have continued to evolve while not being viewed, in a way that could not be simulated except by running the system or something directly equivalent to it. What will be compelling will depend on exact reasoning to get to and flavour of solipsism though.
Intersubjectivity, is useful for understanding how private experiences can be shared. In Yogacara Buddhist thought, also called the mind-only school, the universe is considered to be composed of narrative chains, that causality occurs within these, rather than among mind-independent material. This kind of pansychism still has to explain the regularities we experience of the world, and the metaphor of Indra's Net can help us understand this: as an intersubjective space occuring at the interface between mind and world mutually arising. We can never access phenomena not processed by someone's subjectivity, yet we find evidence for regularities unaffected by subjective processes. Our minds and the world arise and develop together, and become more complex through interaction, especially between minds, through sharing perspectives, intersubjectivity. This helps us understand how meanings can arise that are shared across minds like our own, yet cannot be understood as truly 'out there', or as resulting from a Private Language or from a priori experiences. More here: Is the idea of a causal chain physical (or even scientific)?