Unfortunately, the DAT extension really just defines generic a binary data file. It provides no real indication of the format of that binary data without more context. You did say that the files were obtained via a microPMU so I do know what files you’re talking about, but I still don’t know the format of that data. There are only two tools I know of that can read data from those DAT files.
The first is called Smart Grid Store, which is a cloud-based time-series database solution that uses the Berkeley Tree Database as its storage method of choice. The tool appears to be open source so one could likely determine the format of those DAT files by reading their code. You can see more information about the tool by checking out the SGS website.
The second tool is called the “PSL microPMU Binary File Conversion Utility”. This tool is used to convert these DAT files into CSV files. I have a copy of version 0.1 of this tool and have used it before with varying degrees of success. There was a link in their user manual that would allow you to download the tool, but it appears that they’ve moved things around on their website so I’m no longer able to use the link. If you have access to a more recent version of their user manual, there’s a chance that they have updated the link. You may also be able to obtain the tool by contacting PSL directly.