Anti-Cubes are the opposite of normal cubes in many ways. They are more difficult to find, don't have collectable shards, and are often hidden in plain sight. They are always collected by solving a puzzle of some sort. The only way they are like normal cubes is that there is 32 of them hidden throughout the game.
As mentioned earlier, Anti-Cubes are always obtained by solving puzzles. The puzzles are usually different, but there are a few puzzles that are repeatedly given to you that always give you an Anti-Cube. The list follows.
-Stand on a tuning fork pillar and change perspective based on where you hear the buzzing noises from.
-Stand on a pillar with an O on it and go into first person to read a tetris code nearby and act upon it
-Act upon a tetris code on closed doors you find
There is also one unique puzzle worth mentioning. In the Scientific Zone, you'll find a room with a Clock Tower in it. This room can give you four different Anti-Cubes, based on if the clock's hands are at 12:00. The amount of time it takes for each hand to reach 12:00 is different and is based on real time since you downloaded the game. The amount of time you have to wait for each cube in a loop is 1 minute, 1 hour, 24 hours, and 1 week. This is most likely the one puzzle in the game that will give completionists and speedrunners the most trouble.