1. An apparatus for detecting a malicious domain cluster, comprising: a domain name server (DNS) data collection unit configured to collect DNS traffic over a network and store the DNS traffic in a database; and a malicious domain cluster detection unit configured to generate a domain cluster based on the DNS data, learn characteristics of normal and malicious clusters in the domain cluster, and detect whether the domain cluster is malicious based on a result of the learning, wherein the malicious domain cluster detection unit is configured to comprise: a clustering module unit configured to generate the domain cluster by grouping domains, exhibiting group activities, into the domain cluster based on the DNS data; a labeling module unit configured to assign a malicious or normal cluster label to the generated domain cluster; a characteristic extraction module unit configured to extract a cluster characteristic different with respect to the malicious and normal clusters based on the generated domain cluster; a learning module unit configured to learn the malicious and normal clusters based on the cluster label and the cluster characteristic; and a detection module unit configured to detect whether the domain cluster is malicious based on a result of the learning of the learning module unit; and wherein the characteristic extraction module unit comprises: a domain age extraction module unit configured to extract an average of domain ages within the domain cluster and a standard deviation of the domain ages as a characteristic item; a domain popularity extraction module unit configured to extract an average of domain popularities within the domain cluster and a standard deviation of the domain popularities as a characteristic item; a resolved IP address extraction module unit configured to extract resolved IP addresses of the domains of the domain cluster as a characteristic item; and a domain link extraction module unit configured to extract an average of web page links indicative of the domains of the domain cluster and a standard deviation of the web page links as a characteristic item.