Using sed or VIM to replace space with new line

I have the following data.

1455931_at Chrna3 1420468_at Asb17 1445520_at −−− 1436717_x_at Hbb−y 1431788_at Fabp12 1458975_at −−−

With sed or VIM editor how can I change it to

1455931_at Chrna3 
1420468_at Asb17 
1445520_at −−− 
1436717_x_at Hbb−y 
1431788_at Fabp12 
1458975_at −−−

So all the word that has _at will be the first of every line. Every line consist of pairwise _at and gene terms.