There are two different versions of this problem, and they need different answers.
If you have never installed anything that renders Markdown, Space showing raw text is not a fault — macOS has no Markdown preview generator of its own, so the plain-text previewer handles the file and shows you the source. How Markdown Quick Look works covers that case.
This page is the other version: you did install something, and Space still shows asterisks. Work down the list in order — it is roughly ordered by how often each one is the actual cause.
1. The app has never been launched
macOS registers a Quick Look extension the first time its containing app runs. Dragging the app into place is not enough; downloading it certainly is not. Open the app once, quit it, and try Space again. This is the single most common cause and it takes five seconds to rule out.
2. The app is not in /Applications
Extensions inside apps running from ~/Downloads, a Desktop folder, or — very commonly — a still-mounted disk image are routinely ignored. Move the app to /Applications, launch it again from there, and eject the disk image if one is still attached.
3. The extension is switched off
Open System Settings → General → Login Items & Extensions and find Quick Look in the list of extension points. Every app offering a preview extension appears there with a switch. Confirm yours is present and on.
If the app is not listed at all, macOS has not registered it — go back to steps 1 and 2. Some apps include a button in their own settings that opens this pane directly, which saves hunting for it.
4. Two extensions are competing
If more than one installed app claims Markdown, they compete for the file type, and which one wins is not something you control directly. The symptom is inconsistency: rendered previews sometimes, raw text other times, or a preview that does not look like the app you expect.
Turn the others off in the same Quick Look list, leaving exactly one enabled. This applies to editors and note apps too — plenty of them quietly register a preview extension you never asked for.
5. You are looking at the preview pane, not Quick Look
These are two different surfaces and they do not always behave the same way. The Finder sidebar preview (View → Show Preview) can show a file icon and a metadata list while the Space-bar panel renders the document correctly.
Before concluding anything is broken, press Space on the file and look at the panel that appears over the window. If that renders, the extension is fine and what you were looking at was the sidebar.
6. The file is not actually Markdown to macOS
Extensions are registered against content types, not filenames you find plausible. A file saved as notes.md.txt, or with no extension, or as .mdown / .mkd depending on what the app declares support for, may not be a Markdown file as far as the system is concerned. Check:
$ mdls -name kMDItemContentType notes.md
kMDItemContentType = "net.daringfireball.markdown"If that comes back as public.plain-text, or as a dyn.… identifier, the file is not Markdown as far as the system is concerned — and no amount of reinstalling the extension will change that. Rename it to a plain .md and check again.
7. The cache is stale
Quick Look caches previews. After installing or changing extensions, force it to reload:
# Reload the list of preview generators
qlmanage -r
# Clear cached previews
qlmanage -r cache
# Render one file the way Quick Look would, and watch for errors
qlmanage -p README.mdThat last command is the useful one for diagnosis: it bypasses Finder entirely. If qlmanage -p renders the document but Space does not, the problem is Finder state, and logging out and back in usually settles it.
8. Gatekeeper has quarantined the extension
An app downloaded outside the App Store carries a quarantine flag until macOS is satisfied with its signature. Unsigned or improperly notarized apps can install, launch, and still have their extension silently refused.
If you downloaded the app directly, open it from Finder once with right-click → Open, accept whatever prompt appears, then re-check the Quick Look list in System Settings. If the extension never appears, that is your answer — the app is not being trusted, and an App Store or properly notarized build avoids the whole category.
9. It renders, but badly
Worth separating from "not working": a preview that appears but drops your tables, shows Mermaid blocks as code, or renders images as broken icons is a capability problem, not a registration one.
The images case is not a bug in any particular app. A Quick Look preview runs sandboxed and cannot prompt you for access to the folder around the file, so images stored alongside the document are generally unavailable to it. Degrading to alt text is the correct behaviour; a broken-image icon is just a less graceful version of the same limitation.
Still stuck
If you have been through all nine and Space still shows source, the fastest remaining test is to install a different Markdown app, launch it, and try again. If the second app renders, the first one's extension is at fault. If neither does, something in your Quick Look setup is broken more generally — and a restart, which rebuilds the extension registry, is the honest last step before reinstalling anything.
nicemd's Quick Look extension is part of the free tier, installs through the App Store, and needs nothing more than being launched once. Its settings include a button that opens the Quick Look extension pane directly, so step 3 is one click rather than a hunt.