DACT (Dynamic Adaptive Compression Tool) is a file compression utility, and associated compressed file format, developed primarily by Roy Keene.
It was originally named DCL, but was soon renamed to DACT.
At least for version 0.8.42, files start with signature bytes
0x44 0x43 0x54 0xc3.
There is some minimal format information in the Docs/file_format.txt file included with the source code.