In addition, many theories have knowledge gaps that scientists are happy to temporarily plug with placeholders, in the expectation they'll eventually be filled

In addition, many theories have knowledge gaps that scientists are happy to temporarily plug with placeholders, in the expectation they'll eventually be filled in with permanent replacements. Dark Energy and Dark Matter are two such examples. They're just keeping the seat warm until we actually have an idea what the hell is going on. A particularly long lived example was the heredity placeholder in Darwinian evolution that survived nearly a century before Rosalind Franklin et al., finally filled in the gaps.