We examine possible mechanisms in nature. We’ve observed that life on Earth has changed over time in patterns that tend to similarly track along temporal, biochemical, genetic and morphological characteristics. That, coupled with our understanding of reproduction & genetics, provides very strong support for the conclusion of common descent via modification of existing organisms.
We have not independently confirmed the existence of a cosmic or biological designer. While it’s possible that some ‘intelligence’ intervened in the development of life, it’s not clear that it’s necessary to invoke such an unprecedented mechanism (scientifically-speaking) with indeterminate capabilities. Granted, there is no perfect, fool-proof way of knowing when to consider alternate explanations. However, scientists tend toward favoring the more parsimonious models (related to Occam’s Razor).
We do know that historically, design explanations were proposed for the patterns of life observed. In fact, they were the default explanation for millennia. However, they were generally seen as leading to dead-ends, or unresolvable, or indeterminable hypotheses, particularly as our understanding of nature increased. Consequently, evolutionary models (descent with modification at their core), were seen to be much more tractable and capable of evaluation. It provided a positive research direction that also seems to have borne out over time. This doesn’t mean that it’s ultimately 100% correct or that a ‘designer’ may not have directed the path of life over time. Our understanding definitely has a long ways to go. However scientists have pretty good ideas about where additional knowledge must be extended.
At this time, ‘design’ interactions aren’t known to be necessary mechanisms. We have little agreement within science about whether a designer was present on Earth over the course of the past few billion years. Overall it’s not seen among scientists as likely to be a fruitful research program at this time.
It’s not that much different from how research proceeds in engineering.