Science aims at rationally explaining observed phenomena. The first step in the process is the framing of a hypothesis. For example if you observe that heart patients tend to have high blood pressure, you would form the hypothesis that hypertension is associated with heart disease and proceed to establish its veracity. However, proponents of the "Null hypothesis" claim that you should seek to nullify the opposite of the actual hypothesis - in our example "Hypertension is not associated with heart disease".
All of this boils down to the philosophical basis of the ambit of Science. According to some philosophers such as Karl Popper, Science is only concerned with falsifiable claims. In other words, Science can only tell us that certain facts are not true. The advancement of Science has really been through falsifying established theories. For example the theory of Relativity disproved Newton's classical laws of motion. Till such time, we held these laws to be undisputed truths. The same applies to other laws of Science - the law of conservation of energy and conservation of mass - all of which were subsequently disproved. The alternative theories or even "laws" offered in their place aren't really true. They just have not been disproved yet.