  The identification of universal properties from minimally processed data sets
is one goal of machine learning techniques applied to statistical physics.
Here, we study how the minimum number of variables needed to accurately
describe the important features of a data set - the intrinsic dimension ($I_d$)
- behaves in the vicinity of phase transitions. We employ state-of-the-art
nearest neighbors-based $I_d$-estimators to compute the $I_d$ of raw Monte
Carlo thermal configurations across different phase transitions: first-,
second-order and Berezinskii-Kosterlitz-Thouless. For all the considered cases,
we find that the $I_d$ uniquely characterizes the transition regime. The
finite-size analysis of the $I_d$ allows not just to identify critical points
with an accuracy comparable with methods that rely on {\it a priori}
identification of order parameters, but also to determine the corresponding
(critical) exponent $\nu$ in case of continuous transitions. For the case of
topological transitions, this analysis overcomes the reported limitations
affecting other unsupervised learning methods. Our work reveals how raw data
sets display unique signatures of universal behavior in the absence of any
dimensional reduction scheme, and suggest direct parallelism between
conventional order parameters in real space, and the intrinsic dimension in the
data space.
