A .dcm file is a DICOM file, the format medical scanners use for CT, MR, X-ray and ultrasound images. Your computer has no built-in program for it, which is why double-clicking does nothing useful. The quickest way to see it is to drop it below.

Open it in three steps

Drop the file on the box above, or choose it. It opens in your browser and is not uploaded: you can check that in your browser's network tab.
If the picture is too dark, too bright or flat, change the window. Drag on the image. Left and right change the width (contrast), up and down change the level (brightness). Window and level basics explains why scans need this.
For a series of images, select all the files together. They are stacked by instance number; scroll or use the arrow keys to move through them.

The same works on a phone or tablet in a modern browser: choose the file from the Files app or your downloads. Very large series may be slow on a phone because everything is decoded on the device.

Is it really a DICOM file?

A DICOM file has the four letters DICM at byte 128, after 128 bytes of padding. The viewer checks this and says “Not a DICOM Part 10 file” if the marker is missing. On macOS or Linux you can look yourself:

dd if=file bs=1 skip=128 count=4 2>/dev/null prints DICM for a DICOM file.

Files from discs often have no extension, or a name like IM000001. That is normal; see the DICOMDIR guide for how a disc is organised, and what is DICOM for the format itself. A file called DICOMDIR is an index, not an image: open the image files next to it.

If it will not open

What you see	What it usually means
“Not a DICOM Part 10 file”	The file is not DICOM, is damaged, or is an older format without the DICM marker. Check the source; a zip must be unzipped first.
“Unsupported transfer syntax”	The pixels are compressed in a format the viewer cannot decode (it handles uncompressed, RLE, baseline JPEG, JPEG 2000 and JPEG-LS). The tags still show. See transfer syntaxes explained.
No pixels, only tags	The object is not an image: a report, a structured dataset or a directory. Use the tag viewer to read it.
All black or all white	The window does not match the image. Press Reset, or set a window by hand.
Image looks inverted	The file is MONOCHROME1, where low values are bright. The viewer inverts it correctly; a different program may not.

Other ways to open DICOM files

Desktop viewer programs. Free and open-source DICOM viewers exist for Windows, macOS and Linux; they are better for large studies, 3D and measurements. Choose one whose source or licence you can check, and keep it updated.
The viewer on your disc. Many patient discs include a small Windows program that opens that disc only. It is not needed to read the files, and you should only run programs from a source you trust.
Your hospital's patient portal may offer a link that opens the images without any files. Ask the imaging department.

Avoid pasting patient files into random online converters that upload them. Medical images contain the patient's name and ID, and an upload is a copy you cannot recall. If you need a picture rather than a viewer, convert to JPG here, also in your browser.

Limits

This is a simple viewer: window/level, zoom, a stack, tags. It is not a diagnostic tool and does not do 3D, measurements or overlays; seethe medical disclaimer. Do not use it to make clinical decisions.

Sources

DICOM PS3.10 chapter 7, DICOM File Format (128-byte preamble and the DICM prefix)
DICOM PS3.10 section 8.2, File IDs (why disc files have short names)