Why is the Slack DM list incomplete

I'm experiencing this issue as well. I'd hazard a guess this is a bug with the application itself and has to be filed and solved as such.

I've contacted Slack and they sent me this reponse (emphasis mine):

Sorry to see you have been experiencing this issue with excessive instances of Slack. This issue came to light after we updated to version 2.0, and while the engineers are working on a full resolution for that at the moment, they've put together a support build in the meantime which should fix this up for you. You can access that here: https://slack-ssb-updates.s3.amazonaws.com/support-test/SlackSetup.exe

I've installed the quick fix which minimizes the problem by collapsing all the items in the alt-tab menu down to only 1 lingering notification.

Until then, the only workaround to get rid of all lingering items seems to be to close the application and restart it.

Apart from that we'll just have to wait for a new version.

In order to make slack dark you can do the following for OSX (as of today 7/30/2018).

1) open the following configuration file in a text editor such as sublime: /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js

2) Add the following code at the end:

/* Slack Dark Theme */
document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'https://gist.githubusercontent.com/Icebird/638220c4b3eb7f68c39431740fbd7eb7/raw/6565832413fb971a34d9cb1b23bf0f633504072c/slack-dark.css',
   success: function(css) {
     $("<style></style>").appendTo('head').html(css);
   }
 });
});

3) Re-open slack.

When I did this, I saw that the message backgrounds were black but the sidebar was a purple color I didn't really like. To fix that aspect of it I went into the slack Preferences option when clicking on the little down caret on the top left of the app then going into Sidebar option and scrolling down to themes and clicking Ochin or Work Hard...