Domino

The excluded domino is number 0 and 0

By systematically cataloging all visible dominoes in the image and comparing against the full standard set of 28 tiles (all combinations from 0-0 to 6-6), the domino [0,2] (blank-two) is the one not present in the image.

By systematically cataloging all 27 visible dominoes and comparing against the 28 tiles in a standard double-six set, the domino [2, 3] (or [3, 2]) is not present among the visible tiles. All other combinations of numbers 0-6 appear in the set.

In a standard double-six domino set, each number 0-6 should appear exactly 7 times across all 28 dominoes. Analyzing the 27 visible dominoes: [6,6], [0,1], [4,5], [2,5], [3,3], [1,2], [1,4], [3,6], [1,6], [0,0], [3,4], [0,4], [2,2], [1,1], [4,4], [0,5], [1,5], [2,3], [4,6], [5,6], [1,3], [0,2], [2,4], [3,5], [0,3], and others. Counting occurrences: 0 appears in [0,0], [0,1], [0,2], [0,3], [0,4], [0,5] (6 times). 6 appears in [6,6], [1,6], [2,6], [3,6], [4,6], [5,6] (6 times). All other numbers 1-5 appear 7 times. The only domino containing both 0 and 6 is [0,6], which would account for the missing occurrence of both numbers. Visually, there is no domino displaying the combination of a blank side with a 6-pip side (arranged as two columns of three dots) in the image.