Think outside the box? Indeed. But to add balance to that, one should not in the process forget what the inside of the box looks like as well. Those who are best at thinking outside the box do it not to puff themselves up, but to see how small they really are. As a contented fish in its fish tank appears to have a small, boring existence to us, imagine a larger, more perceptive kingdom (even by scientific taxonomy) to whom our contented existences may appear to be small and boring. This is where true creativity and massive perceptive abilities spawn a sense of intellectual humility; the kind which God adores.
For God to prove himself on demand, physically, would be a grave disappointment, and the strongest Christians should be considerably grateful that he chooses not to do so. The skeptic endlessly demands proof, yet God refuses to insult the true intelligence of man, the "6th sense", the chief quality, the acumen which distinguishes man from the rest of creation, faith.
In response to the opening rhetoric, I will raise the alternative, "Who are you to say that there is nothing more than the universe?" Would not a fish be a fool to assume that all existing entities exist within the fish tank merely because that is all that he can perceive? Imagine the reality of a fish, next, imagine the massive world that surrounds the fish tank. Now, recall our own reality and consider the massive universe that surrounds the earth. Fish have a much shorter lifespan than humans but are content having no perception beyond this, and are confined to a much smaller space but are content having no perception beyond this. In other words, we do exist beyond the fish's space and time as perceived by the fish and the fish alone.
The given premise is based on what we do know about the brain capacity of a fish. We know that animals "higher up" have more advanced cerebral cortices, such as humans and chimpanzees as opposed to frogs and fish. The level of brain complexity plays an important role as to what we are content with in our existences.
Again, suppose there is a kingdom (even by scientific taxonomy) far beyond us and our current perception who, on average, lives 10x longer, is 10x larger, has 10x more space on their planet, and is 10x smarter (more complex brains). In our cluelessness of this, we are perfectly content with our "small" existence, whereas from their perspective we are the "fish". This is also a touch on the saying "ignorance is bliss". It basically sums up the statement about the contentment of fish, and can be applied to most of the animal kingdom. Animals in the zoo can be depressed in their confinement and long to play in the open fields, however they are more intellectually complex than fish. Even higher on the intellectual scale, humans are certainly depressed when locked in, and we tend to want even more than open fields but also to travel.
Figuratively, how could one ever prove to a fish human reality, or better yet, how could one prove human reality to bacteria? It is a being far beyond the microscopic, bacterial perception. Other than the fact that it is incomprehensible to bacterial cognition, it would also be extremely difficult if on their own volition, "being" remained limited to their perception of being. In comparing the universe to a fish tank, God would be much more great in size thus beyond our space and time having created such a massive universe.
Are there any defining factors that prove the human reality is existentially exempt from that of a fish's reality (but simply on a larger scale than fish)? One could respond under the notion that scientific perception defines "being", yet in that case, one is precisely exhibiting the aforementioned fish tank analogy. A fish conjecturing that the contents of its tank are all that exist because the contents of its tank are all that it can verify by means of its own analysis is in denial of acknowledging its limitations. One can believe that the fish is a fool when superior to its tiny existence, likewise, one is also truly analogous to the words of God, "The fool has said in his heart, 'There is no God'," (Psalms 14:1). In the same manner, God is superior to the human existence. I want to note again that the analogy of fish in a fish tank is not meant to be taken literally but rather in a metaphorical sense. Having said that, if fish did have the mental capacity to either believe in a higher power or deny a higher power, and this belief varied among fish, it can figuratively correspond to that of humans on a less intellectual level. We know that fish have limited cognitive reasoning behind the things they physically see. This means that they either could or could not comprehend our existence, even after they see us through the tank, when consuming the food dropped into the tank. However, we have more advanced capabilities, such as the ability to reason, the ability to comprehend the vitality of faith; thus we can subsequently operate with more than physical senses. God tests, but he does not tempt. Indeed all of humanity would believe in God, although not necessarily love God, if he could physically see God waving from the heavens, but it is quite complimentary that God gives more credit to man, his creation, than to thoughtlessly drop food from the skies in order to be comprehended.