In science, the null hypothesis is the opposition to whatever is being tested. For example, when we refer to "dark energy" we are talking about a force that scientists believe is responsible for keeping the galaxies from tearing themselves apart. The null hypothesis then is that without this energy or matter, the galaxies could not exist. This proposition (as demonstrated mathematically) is the only reason that scientists are trying to discover energy that they know nothing about.

Studies on intercessory prayer have determined that there is no significant difference between a person's health when others pray for them and when they don't. With no significant difference between them, the determination is that all results fit the null hypothesis. But this doesn't persuade the believers because they don't understand that demonstrating the null hypothesis with no discernable difference to observation, probably means it's correct.

There seems to be some confusion as to what a null hypothesis is. The null hypothesis for a statistical evaluation is not the definition of a null hypothesis, it is the null hypothesis for statistical evaluation. Just like the null hypothesis for dark matter only applies to dark matter. Please see this link for further information:

https://theconversation.com/explainer-what-is-a-null-hypothesis-10757