Michael is right. More precisely:
The RAW image contains two images: the embedded JPEG preview, which has your camera's processing applied including noise reduction, and the RAW data. When you open your image with an image viewer, you usually see the JPEG preview. In darktable, by default, you will see the JPEG preview in lighttable mode before you start editing the image. In darkroom mode, you will see the result of darktable processing the image, which by default does not do noise reduction. After editing in darkroom mode, the thumbnails in lighttable mode are computed by processing the RAW image.
Activating "denoise (profiled)" is the simplest way to reduce noise. Basically, you trust darkable to apply the right amount of denoising based on your camera and ISO setting (if your camera is supported). There are many other ways to denoise a picture in darktable. Read the manual about the correction groupmanual about the correction group for details.
You can make the denoise module a preset if you want darktable to automatically apply it to all images you open.
