Domino

The excluded domino is number 3 and 3

By systematically identifying all 27 dominoes present in the image and cross-referencing against the complete set of 28 standard double-six dominoes, the domino [0, 1] is the only one missing. All other combinations from [0, 0] through [6, 6] are accounted for in the image.

After listing all 28 tiles in a standard double-six set and comparing them to those present in the image, the [6, 6] domino is the only one missing.

I systematically identified all 27 dominoes visible in the image and compared them against the 28 tiles in a standard double-six set. After cataloging all visible dominoes, the 0-3 tile was the only one not present in the image.

Scanned the 27 tiles and every unique pair from 0–6 appears except the double six tile (6,6) which is not present.

After carefully analyzing all visible dominoes in the grid, I identified the following dominoes present: (3,3), (3,4), (0,2), (3,5), (1,3), (0,2) in row 1, (2,3), (1,4), (1,3), (2,4), (1,1) in row 2, (0,0), (4,4), (1,4), (1,3), (1,2) in row 3, (2,4), (1,2), (2,5), (2,3) in row 4, (0,0) or (0,1), (3,4), (0,2), (1,1), (0,3) in row 5, and (1,2), (2,4), (2,3) in row 6. By checking against the complete double-six set of 28 dominoes, and noting that no domino in the visible grid contains 6 dots on either half (no (0,6), (1,6), (2,6), (3,6), (4,6), (5,6), or (6,6) appear present), and considering that all other low-numbered combinations appear to be represented, the missing domino is (0,6).