In this article, we introduce the notion of a curved absolute $\mathcal{L}_\infty$-algebra, a structure that behaves like a curved $\mathcal{L}_\infty$-algebra where all infinite sums of operations are well-defined by definition. We develop their integration theory by introducing two new methods in integration theory: the complete Bar construction and intrinsic model category structures. They allow us to generalize all essential results of this theory quickly and from a conceptual point of view. We provide applications of our theory to rational homotopy theory, and show that curved absolute $\mathcal{L}_\infty$-algebras provide us with rational models for finite type nilpotent spaces without any pointed or connected assumptions. Furthermore, we show that the homology of rational spaces can be recovered as the homology of the complete Bar construction. We also construct new smaller models for rational mapping spaces without any hypothesis on the source simplicial set. Another source of applications is deformation theory: on the algebraic side, we show that curved absolute $\mathcal{L}_\infty$-algebras are mandatory in order to encode the deformation complexes of $\infty$-morphisms of (co)-algebras. On the geometrical side, we construct a curved absolute $\mathcal{L}_\infty$-algebra from a derived affine stack and show that it encodes the formal geometry of any finite collection of points living in any finite field extension of the base field.