We solve a conjecture raised by Kapovitch, Lytchak and Petrunin in [KLP21] by showing that the metric measure boundary is vanishing on any $${{\,\textrm{RCD}\,}}(K,N)$$
RCD
(
K
,
N
)
space $$(X,{\textsf{d}},{\mathscr {H}}^N)$$
(
X
,
d
,
H
N
)
without boundary. Our result, combined with [KLP21], settles an open question about the existence of infinite geodesics on Alexandrov spaces without boundary raised by Perelman and Petrunin in 1996.