There is no real "dark matter" in a Thunderbird profile. All files should be visible in Windows File Explorer as long as it is set to show hidden files.

My first thought is that you have deleted many messages in but have not compacted the folders within Thunderbird. You may or may not be aware that when you delete a message, it is marked as "deleted" and removed from view, but it still exists within the folder file, taking up space. It's only when you Compact the folder that the message is physically and permanently removed, and the file size will be reduced accordingly.

A factor that some people don't consider is that Sent emails with attachments are stored with copies of their attachments. That could take up a lot of disk space.

Normally, Thunderbird comes out of the box set to prompt you to compact its folders when they reach a certain size. You can change that setting to compact without asking, or to not prompt you at all, in which case the folders will keep growing unchecked until you take action. Those settings can be found in <Tools | Settings | General | Disk Space>. So, have you compacted your folders lately?

As a precaution, I would make a backup copy of my Profile before compacting folders that are extremely unusually large, just in case something went wrong. The backup copy can be deleted afterward.

Thanks Lin (and my reference to dark matter was a joke) - I believe I found the problem. This folder, for some reason, was set as "read only" so I am assuming that this Windows setting over-rode the auto-compact settings in TB (my compacting was set for auto for many years).

Somewhat alarmingly when I tried to turn this off, it would say "read only" was disabled on this folder, but when I checked it again, it was still active. After experimenting and reviewing the permissions etc..., I was able to use a combination of turning off the read only attribute and, before hitting apply, also going in to the advanced tab and making a change there. Once those two changes were selected, I hit apply, and the read only marker disappeared.

Then, the applying attributes windows box appeared, and said it would take "about 10 hours" to apply that attributes to that folder (it was 331 gig). So I anticipate that this will massively reduce that "hidden" disk space usage. (Doing the same on a previous folder resulted in a 1 gig reduction in size in about 6 minutes or so.)

Wish me luck.

Good, I'm glad you found the answer. Folders and files can sometimes wind up with an unexpected read-only attribute if they were ever copied from read-only media, like a CD-R. But whatever, sounds like you're on the road to recovery.

So, whatever it did, it reduced the folder to 147 gig, a reduction of 184 gig and only took about an hour. I think that is still way over large personally.

Investigated further and decided to delet various files in the directory that had multiple iterations with dates appended. For example: Archive (2025_12_17 17_40_02 UTC).msf, and another or several others with only the date/time in the name being the difference. (the inbox and set box e-mail counts stayed the same, I don't think I lost anything) The end result is the formerly 147 gig directory now clocks in at a whopping 2.89 gig. So, down 330 gig plus. Everything appears to function as normal, nothing lost. Good to go.

BUT - I still do not understand why this happened. The files I deleted only totaled 1 gig tops in the File Explorer or whatever the window is called. So somehow there was "dark matter" creeping in somehow.  :-)

Thanks for taking an interest!

LR