Scientific or expert evidence is admissible only if it is relevant to the legally-material factual issues -- only if there is a "fit" between the legally-material factual issues and the scientific or expert evidence.

Crawford v. Washington, 541 U.S. 36, 53-54 (2004) ("The historical record also supports a second proposition: that the Framers would not have allowed admission of testimonial statements of a witness who did....If a declarant is unavailable to testify at the time of trial, a criminal defendant's federal constitutional right to confront, cross-examine, and impeach an adverse witness is satisfied if the defendant had an adequate opportunity in a prior proceeding to confront, cross-examine, and impeach the unavailable declarant.

Huddleston v. United States, 485 U.S. 681, pin? (1988) (sale and possession of stolen property; accused denied knowing the property was stolen; Court assumed that other instances of possession of other stolen....Evidence of a criminal or wrongful act apart from the act that at issue at trial Is not barred by the prohibition against circumstantial use of character if intent, knowledge, mistake, or accident is a legally-material factual issue, the opposing party suggests the non-existence of knowledge or urges mistake or accident as a defense, and evidence of another crime, wrong, or act is offered to prove the knowledge or the absence of mistake or accident by a criminal defendant or other person.

Cf. Old Chief v. United States, 519 U.S. 172, pin? (1997).Background evidence is admissible even if it is technically irrelevant under Federal Rule of Evidence 401 or a comparable State rule.