Domino

The excluded domino is number 0 and 0

After listing all the dominoes present in the image and comparing them to a standard double-six set of 28 tiles, the [0, 5] (blank-five) tile is the only one missing.

By systematically checking all 28 possible domino combinations in a standard double-six set (every pair of numbers from 0-6), the domino [4, 4] is the only one not visible in the image. All other 27 dominoes including [0,0], [1,1], [2,2], [3,3], [5,5], [6,6] and all mixed combinations are present in the displayed tiles.

Scanning the 27 visible tiles shows every pair except the double six. No tile displays six pips on both ends, so the missing domino is [6,6].

By systematically identifying the 27 present dominoes in the image, the complete set of 28 standard double-six dominoes can be accounted for except for the [0|6] tile. The dominoes [0|0], [0|1], [0|2], [0|3], [0|4], and [0|5] are all visible, as are [1|6], [2|6], [3|6], [4|6], [5|6], and [6|6]. The only domino combining 0 and 6 is missing from the arrangement.