There is only what is happening right now in your physical body and your immediate environment. Everything else, including "mental" and "spiritual" realms, is just the brain spinning stories.
Physical reality isn't a theory; it is what you can directly perceive right now through your senses.
It is persistent, verifiable, and exists whether you think about it or not.
Thoughts happen, but the "mental realm" isn't an independent reality.
"Mental" simply means the story-making machinery of the brain—your judgments, memories, predictions, theories, and concepts.
A thought is a real physical event happening in your brain right now, but the content of the thought is usually made up. The content of a thought is NOT made up only when it is a direct, non-interpretive description of a verifiable physical fact occurring in the present moment. Saying "the mental realm is real" leads people to confuse their interpretations with objective facts.
Talking about a "spiritual realm" is almost always an attempt to escape the uncomfortable, messy, physical present moment.
When people talk about spiritual realms, pineal glands, or higher dimensions, they are usually using abstract ideas to avoid feeling physical sensations in their bodies (like fear, anger, grief, or bodily pain) or to avoid being honest with the person standing right in front of them.
If "spiritual" means anything useful it simply means being completely present to raw physical reality without trying to edit, fight, or explain it.