Domination is the fastest-growing field within graph theory with a profound diversity and impact in real-world applications, such as the recent breakthrough approach that identifies optimized subsets of proteins enriched with cancer-related genes. Despite its conceptual simplicity, domination is a classical NP-complete decision problem which makes analytical solutions elusive and poses difficulties to design optimization algorithms for finding a dominating set of minimum cardinality in a large network. Here we derive for the first time an approximate analytical solution for the density of the minimum dominating set (MDS) by using a combination of cavity method and Ultra-Discretization (UD) procedure. The derived equation allows us to compute the size of MDS by only using as an input the information of the degree distribution of a given network.The research on complex networks in diverse fields [1,2], based on applied graph theory combined with computational and statistical physics methods, has experienced a spectacular growth in recent years and has led to the discovery of ubiquitous patterns called scale-free networks [3], unexpected dynamic behavior [4], robustness and vulnerability features [5-7], and applications in natural and social complex systems [1, 2]. On the other hand, domination is an important problem in graph theory which has rich variants, such as independence, covering and matching [8]. The mathematical and computational studies on domination have led to abundant applications in disparate fields such as mobile computing and computer communication networks [9], design of large parallel and distributed systems [10], analysis of large social networks [11-13], computational biology and biomedical analysis [14] and discrete algorithms research [8].More recently, the Minimum Dominating Set (MDS) has drawn the attention researchers to controllability in complex networks [15][16][17], to investigate observability in power-grid [18] and to identify an optimized subset of proteins enriched with essential, cancer-related and virus-targeted genes in protein networks [19]. The size of MDS was also investigated by extensively analysing several types of artificial scale-free networks using a greedy algorithm in [20]. The problem to design complex networks that are structurally robust has also recently been investigated using the MDS approach [21,22]. Despite its conceptual simplicity, the MDS is a classical NP-complete decision problem in computational complexity theory [23]. Therefore, it is believed that there is no a theoretically efficient (i.e., polynomial time) algorithm that finds an exact smallest dominating set for a given graph. It is worth noticing that although it is an NP-hard problem, recent results have shown that we can use Integer Linear Programming (ILP) to find optimal solution [15,19,21]. Moreover, for specific types of graph such a tree (i.e., no loops) and even partial-k-tree, it can be solved using Dynamic Programing (DP) in polynomial time [24].Especially, the density (or fraction) of MD...