A literary, almost poetic descriptor reserved for truly limitless things — 宇宙 (universe), 沙漠 (desert), 海洋 (ocean), 星空 (starry sky). Don't use it for ordinary 'big' things; 很大 or 广阔 fits better there.
memory
浩瀚 (vast, of water) + 无垠 (without boundary). 垠 specifically means 'shore / boundary line', so 无垠 is 'no edge in sight'.
