What is the dark red line on my gums?

Dark red line gums: It may be inflammation, you may have gingivitis or/and periodontal disease. Plaque is a sticky bio film that can be removed with a toothbrush, calculus is a harder and attaches itself to the tooth and root surface and needs to be removed professionally by a hygienist or dentist. See you dentist for an evaluation.

Hard to say: The gums very commonly will have variations of color and texture. If it appears to be throughout your mouth, it is probably the appearance of the blood vessels under the gums. If it is located just in one location, and especially (but not always) linked with some tenderness, it could be signs that something is wrong, like an abscess or irritation with the gum tissue. Get it checked out.

Gingival inflammatio: It could be that the gums are inflamed due to plaque accumulation; or as a manifestation for more advanced periodontal disease. Other factors could be irritation from food or whitening material, or some auto immune disease like lichen planus or pemphigoid.

Gingivitis: A dark red line on the gums is usually indicative of some type of gingivitis or periodontitis (gum disease) and is usually can be corrected with improved home care and professional help from your dentist and dental hygienist.