This puzzle looks a lot like those popular colour-matching grid-based games. Indeed two of the rows in this grid, marked with an X, contain the word PUYO in the centre, suggesting the colour-matching game series Puyo Puyo. This game is one of many that have the mechanic of falling coloured blocks deleting themselves when they form adjacent clusters of four or more of the same colour. The title also supports this theory, the critical mass of an object being the amount/size at which it becomes unstable enough to explode.
Looking at the image, we can deduce the "critical mass" must be equivalent to four balls, or else nothing will happen in our diagram without some sort of undefined intervention. If we play out the game in this way, deleting clusters of four or more adjacent balls of the same colour, and then letting the rest fall down, the chain-reaction eventually stops at this configuration:
Reading top to bottom, left to right, gives the order TURN LEFT. The implication here is that we should return to the starting position and turn the image left before applying the "critical mass" mechanic. In this case, gravity will act as though it is pulling the balls to the left if we look at the image in its original position. Playing through the game again, we get the new configuration:
Again reading top to bottom, left to right, gives the new message EIGHTH STAGE MEAN BEAN. This is referencing another game to employ the "critical mass" mechanic, namely Dr. Robotnik's Mean Bean Machine. The eighth stage of this game is called Grounder, which somewhat describes the way this puzzle has been solved (by pulling the spheres to the 'ground').
|The answer is: grounder|