See explanation below.
Work Step by Step
The stability of a nuclide is determined from the ratio of number of neutrons to protons that is N/Z ratio. If N/Z ratio is either too high or too small then the nuclide is unstable. So 3He2, N/Z ratio is 1/2=0.5 But for 2He2, N/Z ratio is 0/2=0, this shows that 2He2 is highly unstable. This explains why 3He2 is stable and 2He2 is undetectable.