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The fallacy of argument from ignorance
The fallacy of argument from ignorance (sometimes known as demanding negative proof[citation needed]) is a fallacy of asserting that a claim is true as long as it has not been refuted. In other words, X is not proven simply because "not X" cannot be proven. This is related to the burden of proof, because one is placing the burden on the refutation, rather than on the proof of the assertion.

Examples in science
As a general rule, the less coherent and less embedded within conventional knowledge a claim appears, the heavier the burden of proof lies on the person asserting the claim. The scientific consensus on cold fusion is a good example. The majority of physicists believe cold fusion is not possible, since it would force the alteration or abandonment of a great many other tested and generally accepted theories about nuclear physics.