This might be a little late but I found the other answers to be misleading.
It is true that the row class is what is causing the issue, but that is because it is always supposed to be placed inside a container element.
from http://getbootstrap.com/css/ :
Rows must be placed within a .container (fixed-width) or .container-fluid (full-width) for proper alignment and padding.
The container element usually has a -15px padding on both right and left and the row class has a 15px padding on right and left. The two cancel each other out. If one of the two is not present you will see extra space.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…